I have upgraded to FCPX and Lion. Laugh at me if you must but I really like FCPX. I am using a Mac Pro with 12 GB RAM.
I have run into a problem importing some old files which imported perfectly well into FCS 3. One group of files was from a Firestore FS-H200. These were in HDV format but FCPX refused to accommodate them stating that they were missing an atom.
The next lot were old .mov files previously imported from a Sony TRV900 back in 2002. They were standard definition 4:3 files. FCPX did not recognise them as files suitable for import, but good old iMovie had no problems and I imported the iMovie events into FCPX where they play without problems.
The FS-200 files were encoded to ProRes using Aunsoft Media Converter so there is an easy work around but it did take 2 hours.
Have yet to try these files on my iMac which is still using Snow Leopard.

I'd be surprised if it's a Lion Problem.

BTW I Like FCPX too!

Test convert a few files thru Compressor and see if that solves the problems.

I think that there is at least a problem with the Firestore files and Lion as Quicktime player Pro could not open them, but the old standard definition files played normally with the Quicktime player.
There has also been some discussion on the Apple website FCP X discussion board about the FSH-200 problems with Lion.
iMovie also has a problem with these files and an attempt to import into iMovie results in grayed out file names.
I have not tried Compressor as the Aunsoft Video Converter worked for me.

I have come across a few video files that QuickTime Player in Lion will not play the video although the audio is fine. FCP7 plays the same files without a problem. FCPX I have not had a chance to test with these files as they are very low priority. You can also use MPEGStreamclip or VLC to view the files in question.

10.6.8 update messed up a number of older codecs. Mostly ones based on MJPEG-B like M100 lossless codec.

Link:CreativeCOW (http://forums.creativecow.net/thread/4/866019)

This may not have anything to do with your problem but it is a documented problem and it carried on to Lion as well.

No official word from Apple yet when or if there will be a fix.