We already know that HTC has a huge habit of making Windows Mobile smartphones that eventually receive different branding altogether, but now it seems that Asus is getting in on that action as well. The fellow Taiwanese manufacturer, largely famous for its Eee PC line of subnotebooks, is reportedly building a WM smartphone for O2.

We’ll have to chalk this one up to rumor for now, but it seems that a handheld called the Asus GM5 will undergo a bit of a branding makeover to emerge on the other side as the O2 XDA Zest. It is also going by the codename Crystal, though that moniker will probably never make it to the marketplace. Given the XDA naming scheme, it’s almost for certain that this is a WinMo handset. Several previous XDA phones from O2 have been made by HTC.

The skinny on the specs is obviously pretty lacking at this point, but we hear that the O2 XDA Zest will be tri-band GSM, single band UMTS, and it will support video telephony and web browsing.