So far, I've only been able to figure out how to generate PDF email attachments that my users can send from an object by embedding the Visualforce code for the PDF within the <messaging:attachment> tag for a Visualforce Email Template.  The downside to this is that the message body of the email is not editable by the user.  This means my generated attachments always get sent with the same impersonal boilerplate message body.

I didn't know it was possible to do any better until I got my AppExchange newsletter today and saw the demo of Steel Brick's QuoteQuickly app.  I was surprised to see at the 1:30 mark of the video's second part that they claim they are using Force.com's built-in Send Email functionality to send quotes - and then they show the Send Email form load with an editable message body AND with their generated PDF already attached.  

How are they doing that?  Anybody know or have a guess?  

Unfortunately, QuoteQuickly doesn't meet my needs, so I need to figure out how to replicate that functionality.

Much thanks in advance for any assistance!