"i created my account on machine x, now i can't log in from machine y".
Might be an extreme comparison (beg your pardon if I'm being naive), but it's a similar hinderance to using SSH keys. Carry it around on a USB key, or deal with the security outweighing convenience.
I like that. Could even be coupled with a portable browser.
What about determining the salt from the domain for the given login? (perhaps slightly more refined than that to mitigate cases like *.google.com)
I think that's right. It would also need some ability to add mappings like "when I log in from gmail.com, hash like it is google.com"