Douglas Schoen, the Democratic pollster, has a worthwhile take on the national poll numbers. Schoen's article can be found here. Here are the key grafs:
The fact that a Likely Voter sample shows more support for Thompson than a sample of all adults also makes intuitive sense because we know that primary voters tend to be somewhat more conservative than Republicans in general. And, as Rasmussen data has shown, Thompson is perceived by voters as the most conservative candidate seeking the GOP nomination. The implications of this analysis are clear: Fred Thompson is in a much stronger position among the people who will actually decide Republican primaries and caucuses than most observers understand. And Rudy Giuliani's support is sufficiently soft and is made up in part of voters who are attracted to his celebrity and have a much lower likelihood of actually voting.
I'd add one caveat to Schoen's analysis: Conservative voters make up the largest single grouping of Republicans, but there are other groupings--moderates, liberals, and independents--that Giuliani could pool together, combine with those conservatives to whom he appeals, and win a state like, say, New Hampshire. Moreover, Giuliani's celebrity may also expand the electorate, bringing in voters who have not voted in GOP primaries in the past. Still, it's apparent that a large bloc of conservative, likely Republican-primary-going voters do not see Rudy Giuliani as one of them.