When I started using Excel 2007 this morning, I noticed that column headers were showing as 1,2,3 rather than the usual A,B,C.
I successfully googled for the solution and all is now back to normal (by removing the tick from "R1C1 reference style" in Excel Options > Formulas).
Purely out of curiosity, I wonder how this happened because it was something I was unaware of until this morning and I am as confident as I can be that I did not put that tick in there in the first place. Someone (reliable source) sent me a spreadsheet to review yesterday. Could that have been the source?