Log onto or on to
I always hesitate to wade into the tech terms waters because the flame throwers come out waaay too quickly. But since I've had some folks ask through the S.C. Press Association's Editor's 4-1-1 line, here goes:
If you are using AP style or happen to follow Microsoft's Manual of Style you log in to a system, not log into.
Login, logoff, logout, etc., are reserved by AP as nouns describing the process. Microsoft eschews the nouns but favors some of them as adjectives (it never uses logoff, for instance, reasoning that while you might have a "logon password," you don't really have a "logoff" anything).
And checking the number of hits on Google, log on to has more than 9 million vs. almost 3 million for log onto. (I don't ever find this definitive, but offer it up for illustration.)
However, as this 2002 discussion at Fog Creek Software shows, if you are of the technical persuasion, your preferences may be dictated less by stylebooks and more by what operating system or software you favor. It's a pretty complete circling of the subject.