driver.switchTo.frame() is overloaded to accept a frame name or an integer. This int is a 0 based index of the frames available. The first frame would be 0, the second 1 and so on.
I've just run a really quick test using the java binding and Firefox against this HTML page.
<html>
<frameset rows="50%,50%">
<frame src="frame_a.htm" />
<frame src="frame_b.htm" />
</frameset>
</html>
I'm successfully able to use driver.switchTo().frame(0); to refer to frame a and
driver.switchTo().frame(1); to access frame b.