Open the Sharing preferences pane, select "Firewall" and click the "Advanced..." button. Turn on "Enable Firewall Logging" and click "OK." Try to update, then go back to that same screen and click "Open log..." That will show you the firewall log and you'll see if you're blocking the Parallels site.
My guess is that you're not, but rather that their update site is just very unresponsive. (Which admittedly isn't a big help, I know. Sorry.
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Last edited: Mar 11, 2007