Here is what Rikard Skogberg, one of the official SE bloggers, shared with his readers when asked about details around the arc's landing in the U.S.:
I just got something from my US colleagues and the current plan is to have an “a”-variant available at retailers this summer. This version will be optimized for use in the US (GSM/UMTS). I’ll keep you posted as I get more details.
So there you go, a clue from a reliable source that the Xperia arc is already on its way. Unfortunately, “this summer” does not sound as good as a concrete release date would have, but it will have to do until we receive an official announcement from Sony Ericsson itself. If the smartphone does indeed come to the U.S. sporting a GSM radio, AT&T and/or T-Mobile will most probably be the carriers that will offer it. Stay tuned as we will be keeping an eye out for updates. In the meantime, how about checking out this extraordinary Xperia arc commercial?
source: Sony Ericsson Product Blog via SlashPhone