I'm a longtime user of Excel for Windows, but recently a newbie with Excel 2008 for Mac where I found that out the Windows shortcut F4 doesn't work to switch between relative and absolute references when editing a formula. Thankfully Excel 2011 has a Switch Reference toolbar icon on the Formulas Ribbon, as shown below.

Keyboard Shortcut in Excel for Mac
In trying to find a shortcut, I opened up Excel 2008 for Mac, went to the Help search bar, typed in "Excel Keyboard Shortcuts," and was able to find out that Cmd+T toggles the formula reference style between absolute, relative, and mixed.
So I go back to Excel 2011 for Mac and, lo and behold, ⌘+T works there as well.
Gotta love that keyboard shortcut list in Help. It's my new best friend in Excel for Mac. :)