ive had the same problem following my upgrade to Ios5.
everything was fine prior to upgrade.
i upgraded itunes first, then the ipad to the new operating system.
following which i had problems with music and videos being greyed out on the ipad and not playable. i.e. you can see them listed but not do anything with them.
after going through this forum and apple communitys i tryed a number of different things there including changing usb ports, unticking and ticking different options including reduce size of files, ticking some items in music list and not others, undergoing restore, restore from backup etc etc. nothing worked.
last night i tried to reinstall itunes again to the pc, at the end of which it gives the option of repair or remove. i opted for the safe option of repair to start with, but this also did nothing. in desperation i then did the same process again and removed itunes completely. then completely reinstalled again. following which it immediatley started to sync okay, i left it running over night and this morning everything is okay.! thank god, i was really getting desperate.!!
to clarify. after uninstalling itunes it does not remove your music just the program itunes. after reinstalling from scratch, you have to re add all your music back to itunes, this is done by the "add folder to library" option in itunes file (top left hand corner of itunes). then sync as normal.
this update really left me stressed and i was disappointed at the amount of actual apple information available. the update itself without the rectifying of the problems afterwards took approx 1/2 hr for update to new itunes, then 3 hrs for ipad ios5. followed by two days of hell thinking that i woudnt be able to get my music back onto my ipad. finally ive sorted it, without any help from apple i have to say as well.!
furthermore the photos that i had previously transferred onto my ipad disappeared during the process of the ios5 update and did not materialise despite restore etc as did all my high scores on all the games etc. normally it shouldnt be an issue regarding the photos, as they would normally be still on my pc as well, but unfortuantely i hadnt kept a copy there so they are lost for good.
a small price to pay though for finally being able to get back to an ipad that you can actually transfer things onto.!