We are about to buy new hardware to become DB2 (8.2) servers. We currently use an older IBM RS6000 running AIX, but are considering going to AMD Opteron Linux servers, which appears to be a much cheaper option.
We are looking at 2 and 4 processor Opteron(64bit) servers stuffed full of memory with a very good IO system.

Basically the question is does anybody have any experience with this platform. We'd like to know how DB2 will perform compared to a comparable RS6000 AIX box? How well does it scale from 2 to 4 processors? Are there any administration considerations going to Linux from AIX?
Anything else to consider?
Any recommendations on the Linux flavor? We are considering SUSE and Redhat.

Any advice is appreciated.

Linux is a very good platform for DB2 and it performs very well. DB2 scales extremely well on any platform.

If you have Workgroup Edition, I believe the cost for DB2 is the same for up to 4 processors. IBM does not charge extra for daul core processors so that 4dual-cores are treated as 4 processors, even though the system may see it as 8 processors. There are some new dual core Opterons available now (similar to the dual core Athon 64 X2's).

IBM has used Opterons on some of its TPC-H (decision support) benchmarks which you see here:
http://www.tpc.org/tpch/results/tpch_perf_results.asp

DB2 on AIX is very similar to Linux, except that you cannot restore a DB2 AIX backup onto Linux (or the reverse). This is because of BIG ENDIAN vs. LITTLE ENDIAN bit ordering. But you can do a db2move with no trouble.

We use DB2 on Linux here, it's performing beautifully and has been very stable. I can't speak to you're hardware, we're Dell/Intel. But in our case, we haven't had an unintentional outage in over a year.

One caveat with Linux is the flavours... there are so many distrubutions, it makes it difficult for support staff to deal with. Worse, you can actually recompile the kernel yourself, modify its source code (!)... don't do that.

The other thing - if you're used to dealing with AIX, the DB2 support people and the AIX support people are all IBM. In theory that means they can work together more easily than IBM and another vendor. We had some problems getting RedHat and IBM to talk when we had an issue where a Linux bug was causing irratic DB2 behaviour.

Oh, and IBM uses SUSE in the lab... FWIW. I expect that means a tighter relationship.