I just tried 3 different 900SM's and the second the register to the AP they stop responding to any pings and I can't ping anything on the network, pull dhcp, etc. If I fire up wireshark I see plenty of data moving on the network - I just can't talk.
Let me throw this out there! Something very similar happened to me several years ago(never before---or since). In my case it was a faulty CAT5. The CPE would become inaccessible as soon as it would associate to the AP. In my case it was a Deliberant 2.4gHz unit.