Theres no reason why the PR lookup cant be 100% undetectable on Google's end. It's true not everything has been done to fly under googles radar yet, and theres only one real way to garuntee that your not detected, and thats a distributed PR lookup system that hooks into the GoogleToolbar and reports back to a central server the PRs of requested URLs.

That way even when google update the toolbar, you get the right result. And its easy to automate because requests can be made to a central server which then requests the PR of a URL from all the participating nodes. The latency of such a request might be a tad high, but it shouldnt be too bad. Of course if the system got used too much and there where not enough seperate nodes, google could work out what all the nodes where and block them. Simply limiting the number of requests based on the node count would stop that though.

In fact, this idea seems reasonbly plausible if we can work out a way to hook into the toolbar its self.