Found two good tips on http://www.macintouch.com/mail.app01.html#oct8
A) "The workaround for this problem is to make sure that one's attachments are the very last thing in the message. Before attaching files, place the cursor at the very end of the mail message. If there is nothing-- not even a line break-- after the attachment, Mail.app won't create any more inline text blocks, and the recipient should see only the one attachment. "
and B) "zap the resource fork from your file before sending it (using e.g. GrimRipper contextual menu item, found on VersionTracker), Mail should be able to skip the appledouble encoding and send a single base64 encoded data fork (i.e. file)"
Seems to work for me -- and that means I'm still using Mail (and mothballing Eudora)