NzBeagle: As mentioned, the stock charger will output sufficient power for the phone to charge and be used at the same time. It's likely this is the reason, but testing the standard charger will verify this for you.
otari: Use the stock charger, and don't worry it takes a few days for the battery to calibrate and settle in.
Thanks guys, will try it out.
For those interested, I got my case yesterday. Here are some pictures:
DSC_0783 by Hollidog, on Flickr
DSC_0794 by Hollidog, on Flickr
DSC_0792 by Hollidog, on Flickr
DSC_0788 by Hollidog, on Flickr
DSC_0782 by Hollidog, on Flickr
DSC_0779 by Hollidog, on Flickr
DSC_0781 by Hollidog, on Flickr
I think this is a great looking case. Nice texture on the back should give me some confidence when getting used to the size difference compared to my old iPhone. Adds a little bit of bulk but not any that is exceptionally noticeable. All the ports are easy to get into, testing with the stock charger and a couple of other cables with varying connectors, no trouble at all. Tested a couple of pairs of headphones including the stock headphones and some Sennheiser HD400's, no problem fitting those either.
The case has a nice lip around it which you can make out in the photos, this prevents the screen from touching surfaces when it is face down. There is also something similar around the top and bottom edges, though at the top this is more of a pain because the problematic on/off button gets slightly harder to access, as you have to curve your fingertip around it.
Overall, great case, happy with my purchase. Also came with a screen protector which i fitted and then immediately removed, ruined the feel of the screen and my fingers could no longer glide over with the ease they enjoyed sans protector.
Here is the link to a current TradeMe auction for anyone interested in their own. You can search around the internet and probably get it for less, however.