Monaco does not render with anti-aliasing at size 10, though it is AA'd at 8 and at 11 and up. I'm not sure why this is; I poked around a bit on Google to no avail. In Terminal.app there is an option to use AA that seems to override that issue (i.e. it renders 10pt Monaco with AA). Anyhow, I just switched to Bitstream. But having the non-AA as default is probably a bad idea since it looks terrible : ).
Cheers Luke