I am not sure if I can reproce this in all cases, but since the upgrade to gpg4win, gpg-agent seems to hang if I suspend Windows 7 and wake it again. I only notice when I try to view GPG encrypted email via Thunderbird/Enigmail and the passphrase prompt doesn't appear after a few seconds (especially when the message has already been downloaded from the server). At the moment, I work around that by killing gpg-agent, since a new one is started promptly by Enigmail.
By the way: This is when I try to use a key stored on my hard disk. I have never had any issue like this with those keys in previous versions, but I have always had similar problems with keys stored on my smartcard.