You’ll have to change the RepeatLayout property and setting it to Flow will probably yield the result you are searching for.
Take a look at all the other RepeatLayout options here:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.repeatlayout.aspx
Kinda silly the 4.0 framework still defaults the ugly table variant, I mostly revert to the UnorderedList variant.