Update: Alas, no longer — by August 2006, my cell phone was not working with U.S. flight services again, and a reader left a comment to the same effect. It was nice while it lasted.
A bit over a year ago, I wrote a posting linking to a PDF list of local U.S. flight service numbers. At the time, the U.S. 1-800-WXBRIEF number did not work with Canadian land lines or even Canadian cell phones roaming in the U.S., so without a list of local numbers, there was no way to get a briefing or even to close a VFR flight plan in the U.S. using your cell phone — as a result, I always carried a printout of the list in my flight bag.
Recently, the U.S. privatized its flight services, outsourcing them to Lockheed-Martin (though ATC is still public). I decided to try the 1-800 number again, just in case things changed with the privatization. BINGO! The call goes through from my Canadian land line or my cell phone with no problem (and the specialist knew what he was talking about — I take that for granted in Canada, but have not always been able to do so in the U.S.).