Interesting... For as often as I used datediff, I'm surprised I haven't run into this. Your results did surprise me. It appears the key phrase in BOL is:

Is the part of startdate and enddate that specifies the type of boundary crossed. The following table lists all valid datepart arguments. User-defined variable equivalents are not valid.

The URL is http://msdn.microsoft.com/en-us/library/ms189794.aspx

I ran these two queries with different results. The first query returned a 1 as the month crossed the boundary. The second did not and returned zero.

select DATEDIFF(mm,'2010-06-30','2010-07-03')

go

select DATEDIFF(mm,'2010-07-01','2010-07-03')

go

You may want to use "dd" instead, then see if the result is greater than xx days, but that's tricky since the various months have different number of days in them. Sorry I couldn't be more help.

Cindy