When I switched from the Droid to the DX, I just swapped the memory cards (already had the files I wanted on the original Droid). Didn't format. Worked great.
+1 on the formatting. reason i say this is because i took my card out of my xperia x10 and everything was fine till after flashing a few roms. keeping what i wanted and formatting the card fixed all the weird **** that was happining