I see that both the D-Link Canada and D-Link UK sites have put a new 1.20RC075 WW firmware out recently for the A1/A2 HW. There are some bug fixes and several new features, one of them was not clear from the release notes.
Item (10) in the release notes of added features is STP Status with possible values of Enable/Disable, which appears under Wireless Performance settings in the firmware. The online help was not updated to reflect this addition, nor was the users manual which was updated to version 1.13.
I've written to D-Link support but have heard nothing back on this question since early January. Perhaps someone here has determined what this is about or someone from D-Link can comment here in the forum.
I was hoping it might be a Spanning Tree Protocol (STP) related status addition, like most other business Access Points have. But unclear without any documentation.