Yes, mine does the same thing.  I'm not 100% sure about this, but I think
that each MIDI ringtone preinstalled in the 6230 has its own "vibration"
track, so they all vibrate differently.  This way you may be able to figure
out who's calling by how the phone vibrates, although it can be tricky.  If
you download other MIDI ringtones and play them in the Gallery, the phone
won't vibrate, so I think this track has to be specially "composed" and
included in the MIDI file.

Jen,
i THINK it's normal as it does it on my phone too!  I think, however, if
you've set vibrate off in a Profile it wont vibrate unless you're playing
ringtones to see what they sound like...if that makes sense!

All the best