On the older EX250's, you can chop the rear fender off (leave the undertail part to hold all the wires and stuff up there) and move the blinkers (the bulbs) into the smaller red squares on either side of the tail light.
Then get 2 L brackets from home depot to mount the license plate. The rear fairing of the bike should have 2 bolts right underneath that the brackets can mount to.
Some states require the license plate be lit at all times (specifically at night)... the 250's rear tail light has a small rectangle panel on the bottom of it that can be popped out. If your bike is older, this will be a chore so I'd just take a dremel to it. Once the panel is out, cover the hole with some clear plastic, or go ghetto and just use some clear packing tape.
You can see the end result (sort of) in the below pictures. The 250 pictured is a 1996.
I don't know if this mod works on the 00+ 250's, I know it wasn't easy to do on the '03 so we left it alone.
UPDATE 9.3.08 - I found out recently that doing this modification is technically illegal. It's illegal because:
US Code of Federal Regulations, Title 49, Part 571, Standard 108 states that turn signals must be amber or red and must be at least 9" apart, center-to-center. If signals are red, their edges must be at least 4" away from the edge of the tail light.
So if you're going to do this mod, find some replacement amber (or clear if you're going to use amber bulbs) covers for those lights.