For those of you how have Mobipush on your Win Mobile 6 devices here is the reply I got back form their support for the problem I was having:
In Windows Mobile 6 when prompted to enter your username please enter "mobipush\" followed by your eMail address. The domain field should be left empty.
Example:
If your eMail address is "alex@gmail.com", you should input "mobipush\alex@gmail.com" in the username field.