The answer is a big YES. It's been a year since GlassFish had its first production release (V1), where the main focus was Java EE 5 compliance. There was a bug fix release as well immediately after that , called V1_UR1. We are in our way to deliver V2 in a few months and work has already started for V3. Another point to remember is that, although GlassFish V1 was released only last year, the base source code came from Sun's Java System Application Server, which had gone through several release cycles. So, the stability of the GlassFish code is actually pretty good for production use.
BTW, did you look at this link [1] that Alexis mentioned in his response to your question. Take a look at that to get an idea about some real production use of GlassFish.
Thanks,
Sahoo
[1]
http://blogs.sun.com/stories/
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