Yep my Mrs, who uses Macs at work, and a mate that uses some UNIX client both have their emails arrive with attachments for their signature/headers at my Exchange/Outlook work. I always put it down to some incompatibility, but yes could easily see it as MS not playing properly with "open" standards.
One thing to ask, are they using any custom code for their Outlook clients? Just MS has changed/been messing with the way they deal with custom properties:
http://support.microsoft.com/kb/907985
And one of the updates in Office SP3 needs to either be slightly-disabled via the registry or you have to use Redemption to workaround this change
I might have a chat to our Exchange guys and see if there's any server-side options to adjust this sig=attachment behaviour? and have a read thru Tenex's doc links, but sending in RTF with as little non-normal text is the easiest option...
