Ok, having got home, I've done a bit more testing on this, and it *may* be indicative of a wider issue that will affect non-mobile based users.
I can toggle the ability to log into the forums on and off by activating wifi on my phone, which indicates something... odd is going on with how the forum authenticates users.
If this is ip-based, you could have issues with multiple users inside a single NAT-base, such as college/university dorms.