After reading all the posts here, as well as the Pelican water pump removal comments, I purchased a purpose-made water pump removal tool:
http://mysite.verizon.net/res8nm26/bmwtools/index.html I came across this in the Pelican comments - it was made by a father/son BMW owning machinist team. It's an absolutely beautifully made piece - easily OEM quality. Worked like a charm - out in maybe 3 minutes.
After trying a claw hammer without any movement at all, I was hesitant to try the very large pry bar method - there are so many posts where have the pump ended up remaining in the block, and with my engine having 185k on it, I thought the smooth pressure of this tool might avoid this result. And so it was!
BTW - my car was made in 4/98, and the impeller was unlike any other I've seen here - not plastic, not cast metal, but sheet metal pieces tinned or soldered together. The pump had spat out several roller bearings; engine runs soooo much quieter now.