Since this thread seems to be the biggest one regarding this issue with ATV2 Home Sharing issues, I guess I'll add my experience to the pile.
For quite some time, we were happily enjoying our ATV2, primarily for viewing movies on our home theater setup, streamed via Home Sharing from iTunes on the iMac downstairs. Never had a problem with it, until one time some weeks ago when I wanted to watch something, but the ATV2 was unable to see the library on the iMac. We had a gap in our usage, so I can't really pinpoint when this change occurred, but I find it most likely that it's somehow tied to something that changed with one of the ATV2 firmware updates in that timeframe. Of course, it could also be related to a Mac OS update in that period, but that seems less likely.
In all my years of experience with Apple products, this has probably been the most annoying issue I've seen. I'm not incompetent with this stuff. I'm a manager of SW development, am more tech-savvy than probably 99.some percent of the general population, and I know how to troubleshoot stuff, so I'm pretty sure this isn't just a "dumb user" problem. I've spent several weeks now trying to figure this one out, and have searched and searched online for suggestions, and have tried them all, to no avail. Restarts, reboots, enabling, disabling, port opening, reserved IPs, multicast on/off, blah, blah, blah.... Been there, done that, with no success. Well, technically, every once in a while I would be able to see the library on that computer for a few minutes before losing it.
The setup in my situation is as follows:
27" i7 iMac hosting my iTunes library:
> Fully updated Mac OS
> Current version of iTunes
> No additional AV or firewall stuff
> Gigabit ethernet connection to router
D-Link DIR-655 wireless router with latest firmware.
Gigabit ethernet connection from router to gigabit switch at home theater setup.
Ethernet connection from home theater switch to ATV2.
ALL other ATV2 functionality works beautifully with this setup (we also use Netflix on the ATV2 often), so I'm confident there's no fundamental flaw with network functionality, or with the ATV2 itself.
Last night, based on a vague recollection of someone posting something long ago about how he had the same problem, and that the problem was resolved for him after swapping routers, I decided I might as well get myself a more modern router and see if that made a difference. So I went and picked up a spiffy new Linksys E4200, and got it set up in a few minutes. Magically, my ATV2 was able to see the library with no issues. For the record, this was BEFORE I applied this latest ATV2 SW update (4.2.2, I think?). Then I applied the ATV2 update, and things still seemed to be working fine. Since I did this late in the evening, I haven't put much time on the system at all, but I'll be checking it regularly over the next few days to confirm that everything still works.
For some reason in just about every post that I've read on this topic, people don't identify the specifics of the setups they're using, especially the router. I wonder if perhaps many of the people seeing the same issue as me are using D-Link routers, and there's just some obscure incompatibility between the recent ATV2 SW and the way D-Link handles its routing.