SELECT UserName convert(char(10),DateTimeColumn ,111) as groupDate, Count(*) as hits FROM someTable
GROUP BY UserName, convert(char(10),DateTimeColumn ,111)

is the same as

SELECT UserName convert(char(10),DateTimeColumn ,111) as groupDate, Count(*) as hits FROM someTable
GROUP BY UserName, DateTimeColumn

as the convert in the GRUP BY does not really group by that dateformat. (111, whatever that means..).. so the magic is really in the select..

for example try some crazy substringing around your group by to make it nonexistent like:

SUBSTRING(Convert(varchar(max), DateTimeColumn, 103), 0, 0)

it will still do the group by DateTimeColumn, it is the select format that makes the difference!!

Whhhhhhatttttt!