I have no actual plans for this, so I might as well just give it away, maybe someone else will use it for something:
http://www.mindplay.dk/temp/reflections.zip
It's a particle system basic physics - when a particle collides with the surface, the angle of the actual bitmap is estimated, and the particle is reflected upon it ... this means that particles can bounce realistically off any type of surface, they'll even "roll down hill", it looks pretty neat
There are some minor bugs in it still: particles that fly upwards into a surface seem to sometimes disappear - they do not disppear however (they cannot), but something fishy does happen, maybe they come out at the top of the object they hit, I can't tell, but there's something slightly wrong there ... also, some particles seem to bounce directly downwards when hitting a 90 degree vertical surface, I can't tell why this is happening either ...
I'm not spending anymore time on it, but it was a lot of work to get it where it is already - so use this freely, you don't have to credit me for the code, but please notify me of any use.
If you can fix any of those minor bugs, please post your updated version - thanks! enjoy!