Since he endorsed Rudy, and Rudy endorsed McCain, this seems plausible.
Of course, Forbes is a famous flat-taxer, and helped Rudy craft his top-notch tax plan, which included capital gains and corporate tax cuts -- and making the Bush tax cuts permanent.
McCain's plan is now similar to what Rudy proposed, which also leads me to believe Forbes might consider this.
I tend to think that endorsements matter if they mean the endorser will continue advising the endorsee, after the election (should the endorsee win). In the case of John McCain -- who admits he's not an economics expert--its especially important that he listens to the right people.
As such, I would be happy if Forbes did more than just endorse; he should also be giving McCain advice. With fiscally conservative advisers like Kemp, Coburn, and Gramm, supply-siders like me find a bit of solice in the prospects of a McCain presidency. Steve Forbes' support would provide one more good influence for McCain...
You can listen to my interview with Steve Forbes from October by clicking here.