this bug: http://www.simplemachines.org/community/index.php?topic=151616.msg966413#msg966413
still exists in the smf 2.0 beta 3 version of the webinstaller.
It occurs on some ftp servers and it renders webinstall.php unusable.
I'm not sure if the webinstall is fully compatible with SMF2.x either yet. Just run though a regular SMF install.
See: Installing SMF (http://docs.simplemachines.org/index.php?board=2.0;sort=subject)
It should work with 2.0 just fine. I'll look at it and file a bug report as necessary