Happened 2 me on my air-s on about the 3rd ride, heard that evil sound and couldn't work out what it was until the second bearing on the same hub went and the wheel was f*cked...
Got a set of scrub landboard bearings, work fine, just grease em up and push em in (may take a bit of effort).
You can get these bearings off powerkiteshop, coastguard kites etc etc under landboard accesories for a tenner (8 bearings).
WK