Here's the comparison of actual race performance (I happenned to have the historical info in a spreadsheet so just added today's race):
Elite Men average mpm through 40k:
.....2011.2010.2009.2008.2007
#.....40...27...37...41...38
Avg 5:16 5:20 5:23 5:20 5:27
2011 was ~4 second/mile faster than 2010/2008
Top 60 non-elite (since the elites can be more tactical some years).
.....2011.2010.2009.2008.2007
Avg 5:39 5:42 5:46 5:42 5:49
2011 was ~3 seconds/mile faster than 2010/2008.
3-4 seconds difference makes ~1.5-2.0 "assistance" in the overall marathon time - assuming the general caliber of the top 100 runners at Boston has stayed about the same.