It's rather difficult to find information on these; though I did find a whitepaper on this model specifically
http://www.stonesys.com/images/Terminals/Wyse_Winterm/WT-3315SE.pdf
Despite the amount of specification, the version of Windows CE is not listed, but based on the copyright date, it's definitely CE 2.x. Whether it's 2.0 or 2.1x is up in the air though.
Newer models come with newer versions of Windows CE, and some higher-end models even come with a full Windows installation.
I might pick one up; this listing has five.