A big big thanks to Kappaclone for your suggestion! It really helped... Even when I thought my file was completely lost!
So here's another hint for all the rest of you guys: I had the same problem, right after upgrading to iLife'11 I recorded a live performance right from the mixer, it played back alright, so I saved the project. But then I "closed" the lid of my macbook pro (didn't close GB nor shut down the laptop... when I opened back, it was gone: track was there, but you wouldn't listen to it. Tried saving an mp3 version, nothing, it was 115Mb and last one hour, but it wouldn't listen.
I wasn't aware of this bug before that time, so I just restarted GB and my Mac, thinking that it might help.... Then I got the fatal error: The file "No effects#08.aif" was not found.
Thanks toh all your posts I tried many things, restoring permissions and preferences, creating new users. Nothing... Then I started to check all the "Package Contents" for the "trials" projects I had created and I actually noticed that GB had saved my track "No effects#08.aif" within another project I had been just trying out that night (you know, when you do the previous tests just to check everything's alright), not only that, it had saved many "no effects#0x.aif" files into the same "My song" project.
What I did: I copied the "No effects#08.aif" GB was asking for my track and put into the root folder of the project. GB couldn't find it at the beginning, but I click on "Search" (instead of "Ignore") and then it recognized and imported/restored it into its media folder inside the package content. Worked like a charm. Didn't even had to lock the track as Huggart suggested on a previous post.
My suggestion then: check all the other projects/tracks you've been working on before, specially the last one you worked before creating your new GB project. Because you're file might just be there, and not lost at all!
Hope you guys solve your problem... cheers!