Is there a version of BES for Linux?

No.

...and it annoys me, since from what I understand it's a Java based middleware suite, it should run fine on a pair of Xeon CPUs with Red Hat Enterprise. sigh.

I agree (it's annoying)... It's what I was afraid of, but you never know until you ask...

I guess I'm going to HAVE to figure out MS Server 2003 then...

My only advice is to get your partitioning right the first time; the usual tools for resizing things do not work with 2003 partitions for some reason. I have one machine that I misjudged the C: and D: sizes and have no good way to trim a bit here, add a bit there now that it's in production.

(don't use that Dynamic disk crap, it's really custom and if it crashes you can't mount the disk as a normal NTFS volume from like Knoppix or whatever)

My understanding is that RIM is/was working with Novell to port the BES to SUSE Enterprise. I am not sure where that stands now.

Also, you could virtualise the BES (Windows 2003) in XEN on SUSE Enterprise Linux with full support from Microsoft...not sure if RIM will support this, but it does work.

I doubt rim would support that but this thread brings up a good point. It is a shame there is not at least a BES express type package for Postfix.