Update 2010-10-16:
I took pdf-thingy offline from github.
Update 2010-05-08:
Google has decided to discontinue Google wave, so I will stop updating pdf-thingy.
I’ve spent a couple of hours tinkering with pdf export from wave. I put the result here :
pdf-thingy@appspot.com . You can invite the bot to your wave, when you do it will present a link to a pdf version of the root blip. No fancy colors yet 
Somebody pointed me to PDF Wave Exporter, but I could not find the source code for it to tinker with. (I admit that I did not look that hard).
I implemented pdf thingy to learn more about python,wave and google appengine.
I based pdf-thingy on exporty by Pamela Fox , available from google code . This tutorial helped me learn about reportlab on google app engine.
You can find the code here: http://github.com/jacobandresen/pdf-thingy
There are some obvious points of improvement:
- handle text overflow
- supporting annotations
- fancy color templating