Can figure out how to open it.alt text http://img715.imageshack.us/img715/2743/img4094s.jpg Maybe somebody has a similar one.
Might be clips around the perimeter. Try running a butter knife or some kind of shim around the outside to see if you hit anything: if you do try poking in on it.
Try turning the glass and see if it has hooks that hold it in place (kind of like how a smoke detector hangs onto that plate on the ceiling).
The entire visible portion, including the plastic ring around the glass, can gently be lowered -- grab the ring on the outside, gently lift it a bit off the ceiling, ease it away from the ceiling looking for the portion where it slides away along a metal clip. When it is sufficiently lowered from the ceiling, you will have exposed the bulb and can replace it (as I did in mine yesterday, after consulting a handyman neighbor who figured this out).
Mine has spring-loaded clips shaped something like this:
The glass cover hooks onto the base of the clip, and when it's all assembled, the lower wide spots pull the glass cover up firmly against the surround. To open it, I have to pull down firmly on the glass (there are a couple of indentations on opposite sides to give you a grip) to get past the resistance of the first wide part of the clips. Once it's down, the glass hangs from the second wide parts, about an inch below the surround. Then I just undo one of the clips to get at the bulb.
Maybe the outer ring comes off first? Either twist or pull down to release clips. Then you could get at the light cover.
I have this type of fixture over the tub. You have to pull the outside edge of the fixture that rests against the ceiling. Then you have to reach in with your fingers to squeeze the 'triangle shape' wire clips in order to lower the fixture. Once the light has been pulled down far enough, you have to squeeze the wire clip together in order to remove the ends which are inside an opening in the recessed part of the fixture. The ends of the wire is bent to the right and left to secure it against the inside fixture recessed below the ceiling. Its tricky but once you squeeze a clip together you can open enough of the fixture to get at the bulb and remove it. One side is now disconnected from ceiling fixture. You can also do the squeezing of the wire clip on both sides to fully remove the fixture and replace bulb. Hard part is once the bulb is replaced, squeeze both clips so that fit into the opening in the base of the fixture and once in they provide tension and you can just push the fixture back against the ceiling.