For the challenge and to learn more about Bitcoin, I implemented this in pure PHP (bcmath extension required, but I think that's pretty standard. Edit: Goes much faster with GMP extension!)  Please check it over yourself before using it!  I checked several keypairs with Casascius' address utility and they look good, but I'm not an expert, and there aren't real solid tests of the code.  Most of the work is by a pre-written ECC lib I found.

Anyway, the code, with a demo embedded, is at https://gist.github.com/3549107.  It is a little slow right now but could be sped up by using GMP instead of bcmath.  I don't know if anyone still cares or if the bounty has been claimed, but it'd be nice to have.

If there's interest, let me know, and I will extend and/or test it better.

Edit: Cleaned up the code and made it use GMP if possible, as it is much much faster.  Also found ways to use the ECC lib's helper functions more, so there is less code.