Part of my job is to write copy and the use of login comes up quite a lot. I have a feeling this comes up for other folks out there so I thought I'd post the rules and help shed some light on this tricky use of proper web grammar.
Here are the rules:
When used as a verb – it’s two words: log in (to log in)
When used as an adjective or a noun – it’s hyphenated: log-in
o (your log-in password) (your log-in)
You should never use of login.