Dawson also making the ODI squad vs SL means that in addition to missing Somerset away (for the t20s vs SL), he'll also now miss Middlesex, Surrey and Gloucs at home, plus the Surrey four day match at home.
In addition, the fact the he makes the ODI squad vs SL will likely mean he'll also make the ODI and t20 squads vs Pakistan. Which would then mean him missing our other four t20 home games vs Somerset, Essex, Sussex and Glamorgan, plus the Gloucs four day match. All for the sake, based on previous history, of carrying the drinks for England.
Basically, tomorrow is likely to be Dawson's last game in a Hampshire shirt until phase 2 of the county championship starts in late August.