Ultra running definitely does not have the same depth that road and track running does. However, assuming Bekele puts in the training needed for an ultra (BIG if), I think it all comes down to terrain. He's obviously a speed guy, but non-ultra runners consistently underestimate how huge a part terrain plays in these races. Just watch the 2011 World Mountain Running Champs. Around 8:45, all the light speedy people completely break down and you can clearly see Max King and similar strong runners really start to power through. Maybe Bekele could gain this strength, but he definitely doesn't have it, now. vid:
http://world-mountain-running-championships.runnerspace.com/eprofile.php?event_id=4780&do=videos&video_id=53810
P.S. A different handful of 2:15ish roadies men show up to NF 50 mile in San Fran every year thinking they can easily win. They get wrecked every year.