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I don't think that is possible in principle. You'd need to embed some sort of active code (JS, etc) into the email and then have Gmail execute it when the email is presented to the recipient.
If you find a way to do it in practice, then you've probably discovered a very neat exploit.
tags only, not CSS. Does that help at all? :)
If it does the trick, just make sure you've got enough content in the rest of the message - you don't want the hidden text to appear as plain text in the snippet on the summary page...
It's been a while since I've tested the Gmail spam and Adsense filtering , but when I did a few months ago, stuff as basic and totally unsubtle as this worked very well - although they might have tightened things up a bit since.