The IEMobile weblog states that they will be phasing out the old user agent headers for Pocket IE.

The old user agent header (including the current WM 5 device) is:

Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; PPC; 240×320)
Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; Smartphone; 176×220)

The newer devices will ship with the following user agent:

Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile m.n)