The micro plug would be nice as well, but I'm not sure it has enough market share (beyond cell phones) to warrant it. (Despite being higher quality, smaller, and more durable)
If that helps a bit with the debate between Mini vs. Micro USB, here's a quick tally I did by thinking of devices I own.
Inventory of Large USB type B devices:
1 laser printer
If I'd do a random inventory of items that use Mini USB I'd come up with:
4 Cameras
1 Radar detector
3 portable navigation devices (Garmin Nuvi, TomTom, Mio)
2 external hard drives
2 Blackberries
2 Ipod chargers
1 mini speaker system
__
15 devices that I can think of of the top of my head.
Micro USB
1 Droid cell phone
1 external hard drive
_
2 devices that I can think of.
In the end, maybe an online vote might help sway the designers of Arduinos. Please understand that having a large plug is not a dealbreaker, but it is an inconvenience.