Crossgrade Program Mail-In Went Fine
Just thought I'd post about my experience with the crossgrade (FCS Non-UB -> FCS UB) program so that anybody who's still on the fence about whether or not to do it will have a good idea about exactly what to expect.
I'm probably in a minority of crossgrade customers because I don't own an Intel Mac just yet and don't really plan on buying one any time soon. I originally wasn't going to bother with the crossgrade, but changed my mind after I saw some of the benchmarks by final-cut-pro.de et. al. showing that FCP 5.1 is around 25-30% faster at rendering than 5.0.4 even on "old-fashioned" PowerPC Macs, and also has some (admittedly minor) bugfixes and feature enhancements.
Anyway, on to the crossgrade program itself:
For the crossgrade, you have to mail in four install discs (Final Cut Studio, DVD Studio Pro 4, Motion 2, and Soundtrack Pro). Before I sent away the original discs, I burned backups of them in case I needed them while my order was being processed. I would also recommend keeping backups of the original discs because the original FCS apps will run on Panther (10.3.9), while the new UB versions require Tiger (10.4.4). Of course you might never need/want to run FCS on a non-Tiger machine in the future... but then again it doesn't hurt to have the option just in case. Making the backup discs was actually the trickiest part of the whole process... even with the latest firmware updates for my DVR-109 and latest update for Toast 7, burning a DL disc is a bit... erm... touchy

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After the disc backups were made, I made a photocopy of the original discs' faces (as Apple suggests), and packed them up with the completed order form and a personal check for $49+Tax. I mailed them to Apple last Tuesday (5/9) via Priority Mail with delivery confirmation, and the envelope was delivered last Thursday (5/11). I got a shipping confirmation e-mail yesterday (5/15) advising me that my order had been processed and should arrive in 3-7 business days... and was quite pleasantly surprised when my new discs arrived this morning (5/16)

! As others have said, the discs arrive in a non-descript brown padded envelope with an exposed packing list. My particular package was delivered via UPS Ground.
Inside the envelope is... not very much. There are four new install discs (marked 'Crossgrade' in fine print), a sheet explaining that they "are designed to be used with your original Final Cut Studio materials" (i.e. no new printed manuals, although there are PDFs of the new manuals on the first install disc), and a new license agreement booklet. Crossgrade customers don't get a new serial number either; you simply use your old FCS serial number when installing from the new discs.
That's about all I have to report... I haven't had time to install the new software and try it out just yet. Bottom line though is that the mail-in process went very smoothly and very quickly... I was without original install discs for only one week. So to anyone who is concerned about mailing their original discs away (as I was, hence taking so many precautions of backups, delivery confirmation, etc.)... there's probably not a whole lot to worry about (although you might still consider making backups of your discs for the reason I described above).
Hope this helps anybody who's still deciding whether or not to crossgrade!
-Frank