A Blackberry has a USB connection, but it doesn't look like a typical USB connection. It looks like this:
I'm not sure about other cell phones but I would imagine the majority of them do have USB connections considering how media-focused everything has become with cell phones. Here's a blog entry I found while doing a little research for you. http://thebogles.com/blog/2006/01/using-the-blackberry-as-a-tethered-modem/ It gives a basic overview and then has a few links detailing how it's done. The iPod's USB connection (which the iPad will use) looks like this:
To my knowledge, there currently is no cable that could be used to connect the Blackberry and the 30-pin connection the iPad will use. However, I'm sure it's just a matter of time before something comes out. It probably wouldn't be too terribly difficult to do it on your own either.