This is a dumb question, but are you use you're not just over-filling it and letting it leak all over?
The filler/cap is on the passenger side of the engine, and the water pump is on the driver side. The water pump is buried in the middle of the timing belt (which actually drives it) so I hope it's not that.
The filer/cap caps off a crazy piece of plumbing - I don't know what it's called, but it has a five BIG tubes (and some little ones). The big ones are for: TO the radiator, FROM the radiator, one to (or from) the water pump, and two from (or to) the front and rear heads.
So, two of those have radiator hoses attached with clamps. Do you have a loose clamp maybe?
The other three of those bolt onto the engine, and have gaskets. I can't imagine those failing or loosening up, but who knows.
And then you have all the litle hoses - I don't know what the hell those do (some are over-flow / over-fill escapes - maybe all)
This 'crazy piece of plumbing' has several pieces that are bolted together too - with gaskets - those could be leaking. Again, probably not.
Or maybe your 'crazy piece of plumbing' has a big crack in it -
Below are a few pics. If you need that 'cpop', you can have my extra one for whatever they go for.
-AA
(IMG:
http://www.adamanthony.net/mitsu/radCap1.JPG)
(IMG:
http://www.adamanthony.net/mitsu/radCap2.JPG)