Telecomputing has put together a very comprehensive 200-page guide that compares all current thin client hardware devices. They weight the various factors and rank all the different models. They've also posted a PowerPoint presentation (current as of April 2004) that analyzes the thin client device market overall and explains how they did their test.
Note: This article has been updated since it was first published. I initially said that ChipPC had sponsored this study. I said this because the link I found was on ChipPC's website, and ChipPC was hosting the report on their website rather than linking to it on Telecompting's site. It turns out that ChipPC did not sponsor this study, and that it was sponsored by the Swedish government.