Very good question. I discovered that whether or not 0.9 + 0.1 <> 1 is not consistent I guess depending on sequence. If you execute the following in SQL Server 2005, you get "Equal":

declare @f float

set @f = 0.9

set @f = @f + 0.1

if @f <> 1

print 'Not Equal'

else

print 'Equal'