If, like me, you work in an environment where there are many more PCs than Macs, it's useful to always select the following option when sending an attachment via Mail: Send Windows Friendly Attachments -- it's located at the bottom of the Attachment dialog box. However, there is no option in the preferences to do this for every new message. So I thought this was not possible, and that I'd have to choose it every time.
I accidentally discovered that there is an 'Always Send Windows Friendly Attachments' option in the Attachments sub-menu of the Edit menu which does just this. This will strip the attachment of the resource forks, which makes it Windows friendly. Of course if you send this to a Mac, it will still work OK.
[robg adds: I didn't realize this option was there, either, as you can only see the "Always" version in the Edit -> Attachments menu when looking at your main window. If you're already working on an email with an attachment, the sub-menu applies only to that email. Also note that if you send 'traditional' Mac programs to other Mac users, you do *not* want to strip the resource forks. A good number of Mac apps (older ones in particular) still keep a bunch of stuff in the resrouce fork.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041018030308514