Look at the women's race:
1. 2:19:31 Aselefech Mergia ETH
2. 2:19:34 Lucy Wangui Kabuu KEN
3. 2:19:52 Mare Dibaba ETH
4. 2:20:30 Bezunesh Bekele ETH
5. 2:20:33 Aberu Kebede ETH
6. 2:21:30 Lydia Cheromei KEN
7. 2:22:39 Sharon Jemutai Cherop KEN
8. 2:23:13 Atsede Baysa ETH
Then take a look at their iaaf bio's...
Aselefech Mergia
Marathon
2012 2:19:31 Dubai 27/01/2012
2011 2:22:45 Dubai 21/01/2011
2010 2:22:38 London 25/04/2010
2009 2:25:02 Paris 05/04/2009
Lucy Wangui Kabuu
Marathon
2012 2:19:34 Dubai 27/01/2012
This was her debut, but she did run a 1:07:04 Half Marathon in 2011.
Mare Dibaba
Marathon
2012 2:19:52 Dubai 27/01/2012
2011 2:23:25 Toronto 16/10/2011
2010 2:25:27 Frankfurt 31/10/2010
Bezunesh Bekele
Marathon
2012 2:20:30 Dubai 27/01/2012
2011 2:23:42 London 17/04/2011
2010 2:23:17 London 25/04/2010
2009 2:24:02 Dubai 16/01/2009
2008 2:23:09 Dubai 18/01/2008
Aberu Kebede
Marathon
2012 2:20:33 Dubai 27/01/2012
2011 2:24:34 London 17/04/2011
2010 2:23:58 Berlin 26/09/2010
Lydia Cheromei
Marathon
2012 2:21:30 Dubai 27/01/2012
2011 2:22:34 Praha 08/05/2011
2009 2:28:09 Rotterdam 05/04/2009
2008 2:25:57 Amsterdam 19/10/2008
Sharon Cherop
Marathon
2012 2:22:39 Dubai 27/01/2012
2011 2:29:14 Daegu 27/08/2011
2010 2:22:43 Toronto 26/09/2010
2009 2:33:53 Nairobi 25/10/2009
2008 2:39:52 Nashville, TN 26/04/2008
2007 2:38:45 St. Paul, MN 07/10/2007
Atsede Baysa
2012 2:23:13 Dubai 27/01/2012
2011 2:23:50 London 17/04/2011
2010 2:22:04 Paris 11/04/2010
2009 2:24:42 Paris 05/04/2009
2008 2:33:07 Roma 16/03/2008
2007 2:29:08 Istanbul 28/10/2007
2006 2:37:48 Tokyo 19/11/2006
So the top 5 ran a huge PB (or sub 2:20 debut) and even the next 3 ran a PB after years of marathon running on some fast courses.
You have to go to 9th place Mamitu Daska to find someone that had run faster before Dubai 2012.
So did a whole bunch of men and women from different training groups decide to dope on that same day? I'm thinking the course might have had something to do with it... short perhaps, or maybe something else changed from previous Dubai marathons?