The misspelling "alot" became common in the last 20 years or so, I think. Still not an acceptable spelling, as indicated by the Bike Forums spellchecker flagging it as an error. Maybe people have a vague sense that it should look more like the word "allot" (distribute portions or shares).

Spellcheckers can't identify all errors, though. You see people consistently using the spelling "ridding" when they mean "riding," for example.