There may be a few issues:

• Once you have computed a password for a hostname, you cannot change it. There are some workarounds, of course, like saying "hostname1" for then on, but they don't seem attractice.
• If someone breaks your master password, all of your passwords crumble.

But that isn't to say I haven't considered something like this myself; it's probably all-right for the low-security stuff. Were you planning to write this in Perl?