The number input type is for, you guessed it, numbers.
Mobile Safari and Android do not validate, but does provide a number-based keyboard when this input is in focus. Not as limited as the numbers-only telephone keyboard, has more keys available (like a period). iOS 4 will allow anything to be submitted while iOS 5 will remove anything you enter in the input that isn't part of a floating point number (e.g. periods and dashes are OK) on blur. iOS 5 will give you the "numbers only" keyboard (a bit like type=tel) if you use pattern="[0-9]*".
Chrome 10-11 won't allow you to enter any character other than numbers, ".", or "e". Also has increment "spinners". Will allow you to type a period, but not actually submit. Multiple periods will truncate the rest of string and will submit. Chrome 12-14 (and IE 10) will allow you to type non-numeric characters, but they are removed immediately upon blur (and will not submit) which ahears to the spec. Chrome 12-14 will also auto-reformat numbers to include commas (e.g. 2000000 turns into 2,000,000 = bad for using for something like a credit card number). Chrome 15 has stopped reformatting numbers, although things like spaces are still not allowed meaning it's still not a good choice for credit card numbers.
Opera (tested 11) will allow any character. Non-numbers also pass validation. Does have "spinners" (and and down arrows to increment number).
If the min, max, or step attributes are also present, that will validate as well (no lower than min, no larger than max, value % step = 0)
Notice there are no validation message screenshots. Of the browsers that support this, Opera will accept anything and Chrome won't allow you to type anything invalid.
Incrementing works with the mouse scroller (tested in Opera 11 and Chrome 10-17, Safari 5.2).
IE 10 claims to support it but doesn't validate it or provide and UI.