I've been using the Gns Bluetooth gps receiver for a few months. Got it off a us eBay seller named buck for around $80 incl postage.

The gns receiver works perfectly with wifi iPad or iPhone and is significantly more sensitive (32ch vs iPhone a-gps) and accurate than the iPhone built in gps. Works well with all the nav apps I've tested. I've used it with motionx, mud-maps, hema and bitmap. Motionx is my favorite app but the it lacks the ability to import third party maps and pre-downloading their maps can be time consuming, so I supplement it with bitmap and some 1:25k nsw topos I got from somewhere in ozf2 format (which are not as good as the actual scans in mudmap).