PayPal Sandbox's support docs warn against a firewall blocking HTTP POST connections. I called on GoDaddy support, read them the doc, and the rep who handled me confirmed that they are not blocking Paypal traffic in such a way.
For what it's worth PayPal Sandbox's blog announced that there were delays of up to 1 hr processing notification updates that ended 3/13/08... two weeks ago but still useful to know that the sandbox isn't as stable as one would hope.
And a correction... OOPS, user sparky created *this* thread, and AlexStudio created the installation guide thread at http://www.oscommerce.com/forums/index.php?showtopic=177853