Many semi-professionals are using 29ers, to great effect. Just look at the professional mtn biking scene in the UK – it's almost zilch, even Matt Page has a day job.
The road riding example above: extrapolate that and you have some more information so don't be so blinkered, it's 2010 and 29ers are winning all over the country in all manner of races from my local High Wycombe race meet to the Kielder 100. If you race or compete in events, be it Mayhem or your local club events, you’ll see, year on year, more riders commuting across to 29ers, and that’s simple fact.
Out and out speed is really only a small part of the bike riding experience and equation, but since the OP asked specifically about the speed of 29ers, I've confined my comments to just that. Unless you suck-and-see for yourself, you’ll forever be relying upon 2nd hand snippets of information and anecdotes, not the most informed way forward.
Last w/end was the UK solo 24hr mtn bike championships. My two fastest laps were on my 29er. I then had a rear wheel bearing issue so rode the Turner Flux for the remained of the 24hrs. This took in the Red & Blue routes at Newcastleton, so my personal advice would be to ignore the uninformed riders who pedal the line that 29ers are slow in the single track as they're talking from little or no personal experience; take your lead from those who have made the leap and actually own both 29 & 26ers.