This has depended on the particular version. Some do not get updated. PrE 3.0 got one, that updated it to Vista. I do not believe that PrE 4.0 had any. There was no PrE 5, or 6. I do not think that PrE7.0 had any, but then PrE 8.0 had one that fixed a lot of issues with graphics cards of the day. To date, PrE 9.0 has had one update. There may, or may not be another. PrE is on about a 1 year release path, we are early into PrE 9, so there could well be another. We just do not know.
In PrPro, CS2 had none, and cannot recall one for CS3. CS4 had two big ones, and there were several updates to an element in PrPro, AME (Adobe Media Encoder), and also some for Encore, included with PrPro. To date, CS5 has had two updates to PrPro, and some have affected other elements. PrPro is on about an 18 mo. upgrade path, so more time to release updates.
Good luck, and wish that I had some concrete info to offer, but I do not.
The support for this product is amazing as is this forum and such a quick response from Bill Hunt who answered my question to a tee.Thank-you for that.I am sure I will have plenty more in the months to come and maybe answering some questions myself when I'am not so green.
Please feel free to ask away. We will all do our best to get you the right answer. If we can't, then we'll make something up...