According to AlternaGeek, and to a CNET reader, T-Mobile is blocking people from sending Twitter updates via SMS. CNET cell phone editor Nicole Lee was just able to update her account from her phone, so whatever block there is does not seem to be universal (or perhaps it has been lifted). We are looking into this.
A workaround for impacted users would be to access Twitter via the mobile Web site, m.twitter.com.
All users will be kicked offline on Saturday when Twitter performs maintenance on its servers.