It appears to be the preferred spelling of Gmail, AOL Mail, and Yahoo Mail. Hotmail hyphenates. I think the hyphen still prevails in most edited, printed matter. I'm not sure which is more common online, though I'd guess the unhyphenated form is more popular simply because it's easier to type.
Edit: You can't trust those Google numbers. "Email" gets a lot of hits for "e-mail" and vice versa. Actually, I'm not sure why the numbers are even different if it seems to ignore the hyphen.