@Daniel Beardsmore said:
The extension change warning is perfectly valid, but it's a symptom of at least one larger problem. I will work on the assumption that extensions are worthwhile, although I am not sure about this either. The primary problem, and it's wonderful to see that Apple made the same mistake as Microsoft, is that graphical file rename selects the entire name including the extension. Therefore, if you type, or paste in, a whole new name, you automatically wipe out the extension.
Vista actually fixes this issue. When you go to rename a file it selects all but the extension. At least they got something right with it.