Bronson,
I have been writing about issues with text following attachments, even when only the Return key since 10.0, and even blank lines between multiple attachments -- it has been very well documented, by me and others.
Also, a message with Signature is not a Plain Text message if the Signature has content, itself, other than Plain Text. In Panther there was a separate preference for composing the Signature in Plain Text, I think, but in Mail 2.0, there is a convenient option in the pull-down under Format in the Menu Bar to "Make Plain Text" which has been provided for such situations, but perhaps more for sending to AOL, which cannot handle RTF very well, if at all.
What is interesting to me, is that I used have an old PC running Windows '95 set, using a Netscape mail client, and it always handled anything OSX and Mail could throw at it, even when only 10.0, and before Windows Friendly, and other options we have discussed were available.
All the best,
Ernie