<< Is there a reason to hold out from the Tiger MP that I'm not seeing? >>
I'd wait for offerings from other vendors. The Tyan boards seem plagued with problems. Some people have reported having no problems with their boards but many of us have had issues. There are glaring BIOS problems that Tyan hasn't fixed in both the Thunder and the Tiger. (The temperature/voltage readings in the BIOS are wrong unless you hit a key to refresh them - the same bug keeps MBM and other utilities from reading the info properly, for starters.)
Many of us have had to go through multiple board swaps to find one that works mostly properly.
<< If you're just a regular user looking for a boost in multitasking performance, wait until the next speed grade of CPUs comes out and then upgrade to another single processor system >>
Gotta agree with that statement. If you are doing things that make use of a MP system, then fine. (database server, major 3D rendering, etc.) If you are not, you will see much more performance from a faster single processor system than the minor bump a 2CPU rig will give you on everyday, nonSMP tasks.