I have 64 bit Windows 7 and MiniKies seems to work fine up till now, although no updates have come through yet. The full version of Kies is not compatible which is a shame because it seems like a cool program.
My phone says unregistered as well. Phone is not modified. MiniKies is "greyed out" where it says to upgrade firmware. I am guessing the unregistered notice does not affect upgrade. I just keep checking for the upgrade. Would be nice if AT&T would read forums sometimes.