I sure hope we figure this out when the HardSPL gets released. The only reson I would upgrade to a TP2 from my Diamond is because I have to carry around an ATT phone and my Diamond. Otherwise, I'll just wait until a good WinMO phone on Sprint launches with a fast OMAP, Tegra, or Snapdragon chipset.
EDIT: Found this on XDA right after I posted.
http://forum.xda-developers.com/showthread.php?t=548957
Looks like there's a company unlocking the SIM slot on the TP2s. The OP states that he can get the phone to read the SIM cards now, the phone can find the network, but it won't register.
Honestly, reading between the lines, I think he used an unactivated SIM card... so yeah, it kinda *wouldn't* register on the network. :P I could be wrong though.