AutoCAD 2009 subscription customers had a bonus several months ago to download additional tools to use
.pdf files as an underlay. This is a real benefit and shows that perhaps Autodesk finally understands that while we'd love to have the perfect world file sharing that is certainly possible with the electronic age and with Autodesk or CAD products, it isn't the way the world works.
Due to liability issues from what I can understand, many firms will only send
.pdf files and not .dwg files. Sometimes though this is also the least common denominator among general contractors distributing documents to many subcontractors. These general contractors may not be well versed in
.dwg files or CAD. I'll give them the benefit of the doubt.
I spent some time trying to explore this subscription bonus for 2009 only to sadly discover that it was for AutoCAD only and not for the vertical products of AutoCAD Architecture and AutoCAD MEP. For 2010, now we finally have the tools. Here's a look at what you'll see
First, when you go to the Insert tab of the ribbon, you'll see a new Reference Panel.

You can set layers now for your underlays and turn snap on to snap to objects, though I didn't have any luck with the sample
.pdf I tried.

Notice that when you choose to attach a file you have many more files types available now.

Here, I've chosen to attach a
.pdf file.

You won't see this PDF Underlay tab on the ribbon unless you right click on the border of your
.pdf. This shows you several options available.
This is a real plus for AutoCAD Architecture and AutoCAD MEP 2010.
I referenced several blogs in a previous post that discuss features of AutoCAD 2010 in general. If you didn't already come across it, you'll want to read the
AutoCAD 2010 Preview Guide from Heidi Hewett.