This kinda surprised me.. I haven't tried it before, but I attempt to install a 64-bit guest, I choose "SUSE Linux 64-bit" from the list in the VMware Server Console. The console runs on my Windows XP box, and the host is a Dual AMD Opteron system with 12GB RAM.
Anyway, as soon as I choose the SUSE 64bit, a warning show up, telling me that "[b]64-bit guest operating systems are not supported by this host and will not run[/b]".
So, why not? Dual Opterons on the host, the host is running SuSE Linux with kernel 2.6.16.13-4-smp x86_64, I don't think I can get more 64-bit on the host.. 
The VMware Server is build 23869..