Are there speed cameras on stretches of the Autobahn with a temporary speed limit?

EDIT: I've rewritten my answer, especially to address the question about the variable speed limit.

The signs

These signs can show different speed limits, which is for example used as flow control. If there's much traffic or even a jam ahead, speed limit is reduced.

In general, these signs are as valid as their normal, printed counterpart.

Enforcement

Speed limits are of course enforced in Germany, though the fines are relatively low compared to other countries.

Stationary cameras

@Guntram Blohm has already shown some of them. But especially on the Autobahn, I know this ones on the A3 near Limburg:

The speed limits signs were extraordinary large, and repeated three times or so. If you don't care, the last sign will make a photo ;-)

There's also a web site listing all stationary speed cameras on the Autobahn

Stationary cameras on tracks with various speed limits

According to the site posted above, there are indeed some stationary speed cams, which adapt to the variable speed limit. The right column states the threshold for the trigger:

Non-stationary speed cameras

Of course, we have mobile cameras. They can be stand-alone devices, sometimes built into cars parking at the side of the street, or even trash bins (!)

Police

A laser speed gun is often used by the police, but of course not on the autobahn. But they have civil cars (i.e. no police cars) with a video recording system, which is also calibrated for precise speed measurements. This videos are also conclusive when you go to court.

We don't have many of those cars, and they are more or less looking for the really bad guys, which drive waaay to fast or show other bad driving habits.

Special cameras

Just for completeness, and because of interest:

The Autobahn A1 crosses the Rhine in Leverkusen, and it turned out a few years ago, that the bridge is very ramshackle, and even can't be repaired. A new bridge will be build, but it's still in the planning phase. The existing bridge has a speed limit of 60km/h and is closed for vehicles of more than 3500kg. But since the detour is very long, many trucks ignored this. The bridge now has two cameras in each direction, one for speed, and one for weight...

Ah, and there are some pilot projects for cameras measuring the distance between vehicles on the autobahn, because it is often way too low.

Note:

Police will always stop and fine you directly. Any kind of speed camera is operated by the city, district or state, and you'll receive a letter from them, not the police.

As others have noted, these temporary speed limit signs are equally valid as permanent signs are. As such, when speeding and getting caught you face the same consequences as if there had been a fixed sign on the roadside.

However, I disagree with the other answers somewhat. Germany is very lenient when it comes to enforcing speed limits. It is not unheard of for speeding in an urban 30 km/h zone to only be enforced at speeds of 40 km/h onwards. (that’s over 30 % more!) I have heard various rules of thumb as to which faster speed is acceptable, and most boil down to a formula of:

10 % + 3 km/h, rounded up to the nearest multiple of ten.

Attempt that kind of speeding in other countries and you are in for big fines.

Another point is that — despite what many people perceive — speeding fines in Germany are rather low when compared to other European countries. If you are caught driving 140 km/h (measurement tolerances already deducted) on the motorway on a stretch where 120 km/h are allowed, your fine is merely 30 €.

The density of speed cameras varies greatly depending on the Bundesland you are in. Baden-Württemberg is on top of the ladder with over 1100 fixed cameras while Bavaria, despite being the largest state, only has 85. (Numbers from early 2015) The police ‘have better things to do’ in many areas rather than stand around and enforce speed limits. So altogether, there is a strong lenience towards speed limit enforcing.

This means, if the temporary speed limit is 120 km/h, most drivers will drive at 130 to 140 km/h, and even if the police do measure, this is unlikely to get enforced. Go with the flow and typically nothing will happen. If enforcement is present, they will be on the lookout for the blatant speeding outliers.

As pointed out by @sweber these signs are valid and this being Germany, yes the limits will be enforced.

I've been passing through one such stretch fairly frequently and while I have never seen mobile radars there, I have been warned of popular spots and have frequently heard the warnings on the radio. Also the vast majority of drivers really sticks to the speed limit (with German precision) in that area.

The system of temporary traffic controls is used for controlling the flow of traffic around peak hours (in theory, in reality always), alleviating traffic jams and avoiding accidents (i.e. slowing down traffic before hitting a jam / construction site / accident site).
In the aforementioned section there are cameras on poles throughout for monitoring traffic and most people do stick to the limits (within 10km/h at least), so I would be very surprised if there were no controls at all. In fact, if not knowing the place you should always expect mobile controls and drive within 10km/h of the limit). Police cars might try to catch you if you really exceed the limits as mentioned by @sweber (but personally I feel 30km/h over the limit might only trigger them on a bad day).

Finally, here is a map with permanent speed controls. Mobile ones are also announced on the radio (on most stations, in German, every full and half hour after the news) and there exist apps.