On my N900 both password fields are not greyed out. I am also using IMAP.

Perhaps check the settings after a reboot?

That said, the password for SMTP is stored in gconf, at /apps/modest/server_accounts/<account_name>ID1_transport. You can try to change it there using gconftool-2 -s ... no idea if it will really 'stick'

If all else fails, would you consider removing the account and adding it back?