30-June-2011
20:15 UTC
MtGox USD 13363
TH  USD 3341
B7 USD 1938
Source: Bitcoin Watch (volume in the last 24hrs)

MtGox 72%
Tradehill 18%
Bitcoin7 10%

Changed the source to Bitcoin Watch (previously I was using Bitcoin Charts). It is more accurate to calculate ratios out of the volume in the last 24 hrs rather than the volume since midnight UTC, due to the uneven distribution of trading during the day.