Having said all that, I wouldn't recommend a FR-type tremolo for dropped tunings since you'd have to change the trem spring tension every time you change tunings

I think that's the whole point of having the D-Tune switch built into the guitar, but I can't say how good it works because the guitar wasn't in tune to begin with.

unless of course you can dedicate one guitar to each tuning!

Heh, I do exactly that with my acoustics. Of course, using a tuning of DGDGAD is very difficult to get to quickly from a traditional tuning. It just means I have to use one of my Takamines rather than my beloved HD-28. I have one guitar that pretty much stays tuned down all the time so I can just pick it up and practice.

I don't plan on getting into aleternate tunings on the electric though. It's already such a pain on the acoustics!