From the University of Pennsylvania's FactCheck.org, (listen to the ad and read the full analysis):
An Obama-Biden radio ad hammers McCain for being opposed to stem cell research. Not true. Meanwhile two spots from the McCain-Palin campaign, together with the Republican National Committee, describe McCain's support for the research; they're largely accurate.
By saying that "John McCain has stood in the way - he's opposed stem cell research," the Obama ad seriously misstates the view that McCain has held on this issue since 2001, when he began backing embryonic stem cell research, a position that was out of step with that of many of his fellow Republicans.
The McCain/RNC ads would probably lead listeners to believe that Palin shares McCain's views on this topic. That's not true. But we find that to be a minor flaw compared with the misrepresentation in Obama's ad.