Lion does clever stuff in the background to ensure that your files can be "versioned". It's an extension of Time Machine. So, for example, if you open an image in Preview, make a change, that change will be saved immediately. There is an option to step back through the revisions. Files that have not been touched for over two months are locked by the system to prevent editing (If you did need to edit the file, you'd need to go to File>Save As to save a new version).

It could be that this system is being invoked as you copy files to the Lion's drive, and that is changing the modification dates.

A lot of Apple native apps do this at the moment, but you can bet your boots that in the future other apps will do too.

It would certainly point to something happening with Lion, although Snow Leopard also has Time Machine, and it wasn't an issue with that system copying to and from Tiger and Snow Leopard. However, when copying only to Lion, there is an inconsistency to it that just doesn't make sense, with a file having its date changed with one copy attempt, and then the next time it is left alone. It does seem to always retain the date when copied singularly, which just isn't practical of course.

At the moment, I am just looking at one job at a time when I need to work on it, and recopying batches of files until they all retain their original date. My fear is that Lion will do something clever in the not too distant future and change all the dates again without me knowing.