I suspect Remote wipe or "Erase all content and settings" don't really do a full wipe, though they may be good enough for your purposes. I say this because "Erase all content and settings" used to take a little over 2 hours to complete- I know this from experience. I did both "Erase all content and settings" and a remote wipe on my 3GS last week to get rid of a problem that a restore couldn't fix. Both took apparently no time, despite the "two hours" warning before running. Also, there used to be a progress bar to indicate how far in the wiping process the iphone was, but in 3.0 there is no progress bar.
The two methods in 3.0 are identical as far as I know, it's just that one can be run remotely. But to actually wipe the data, the way 2.x did, it simply can't be done nearly instantaneously. It takes time to overwrite the entire flash memory (save some basic operating system components) several times, the way 2.0 did.