The gmail servers do support plus addressing, but as mlongtin and I said, it's mostly the web forms that have problems with plus addressing.
And these are exactly the places you want to use it - to trace who sold you out.
OTOH, it won't be too long before spammer learn about plus addressing and just strip off the plus part.
BTW, with free subdomains, would it help to have one real email address per subdomain and using catchall to forward to one mailbox?
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