If you don't like Kindle and Sony Reader's small screen (most PDF documents/eBooks require 1024x768 resolutions for full page view unless you are using iPhone or Nokia N8xx tablet; Kindle is 600x800), you might want to wait until 2010 for Plastic Logic's reader.
Check out this video:
I bought Sony Reader over a year ago and promptly returned it because it was no good for PDF eBook reading. Nokia N800 worked much better and iPhone's PDF reader app (called Files) is wonderful.
If you wait 3-4 years, you will see color readers (and writers) based on long battery life E Ink technology.
Also check out these readers from iRex technologies. None of these provide the Amazon platform yet but Amazon should allow them rights/access to their store (unless Amazon wants to be in the hardware business).