I don't really have much to add to all the fuss. The origin of "log in" as an idiomatic combination of a verb and an intransitive preposition is obvious. There's nothing unusual in the transformation of this V+P combination into a noun, or in the tendency to write the noun (and sometimes the verb) without an internal space. The list of analogous cases is a long one: "strike out", "show off", "make up" — or "strike-out", "show-off", "make-up" — or "strikeout", "showoff", "makeup". Etc. Nothing to see here, move along please.
I'll note that such V+P combinations are often treated as noun stems ("strikeouts", "showoffs", etc.), but it's rare for them to become verb stems ("striking out" not "strikeouting"; "showed off" not "showoffed"). However, there's a marginal tendency, apparently mostly among semi-literate adolescent gamers, to treat login as a verbal stem that can be inflected. Thus
Let me tell you all a little story. About 3 months ago, I (James94) made a user mad, then I got a 4 day ban. I got impatient so I made another account (Antwan). Then about two days later when I loginned to my new account and I had a message saying that I was banned for bragging too much at brawl. […]
Please unban James94, and you can ban the profile I am loginned on right now (Jmes94).
well my server is truely up and running and i have created accounts and loginned and everything, then i use the mysql query browser and change the GM column value for my char to 1 as i have heard people say but what i really want to know is […]
Or this web forum post on the subject "loginning onto a network server".
I probably wouldn't use "loginned" or "loginning" myself, but not much in the fate of the world seems to depend on the question of whether these usages catch on or not.
Really, the only thing worthy of note in this whole discussion is the fact that it's taking place. This reinforces our usual points about the social psychology of peeving, and the odd mismatch between the popular enthusiasm for linguistic analysis and the lack of competent attention to teaching the relevant concepts and skills.