The ICANN agreements with all the gTLDs call for a ban on two-letter registrations unless specially approved, to avoid potential confusion with country code domains. All other TLD names (com, net, org, museum, etc.) are also barred as second level domains (e.g., you can't register museum.info).
These restrictions have been enforced from the start in new TLDs like .info and .biz (.aero got special approval to allow the use of two-letter airline codes). For existing TLDs, old registrations were grandfathered, but no new registrations of two-letter names were supposed to be allowed since the ICANN agreements were signed a couple of years ago. However, the only one that seems to be enforcing this is .org, and only since it changed to a new registry a few months ago.