MLSsoccer.com posted an article about all of the new players that joined MLS in 2011, and where exactly all of them came from.
It's quite an interesting read, as the article explains which player signings are regarded as domestic and which are regarded as foreign, and the answer is probably a little different than what you may have expected. From the MLSsoccer.com article;
What makes a signing domestic or foreign? The numbers here have been determined by the player's last team before being signed. So an American or Canadian player can be counted as a foreign signing, like in the case of Jay DeMerit or Benny Feilhaber coming from Europe. Likewise, domestic signings include those coming from teams in the American soccer pyramid regardless of nationality, as well as undrafted college players.
So because the last club DeMerit played for before signing with the Vancouver Whitecaps was Watford in England, he is considered a foreign signing. The same for Feilhaber who previously played for AGF Aarhus before joining the New England Revolution.
The article also talks about MLS's weighted lottery, and Generation Adidas players who sign after the MLS SuperDraft. Five of these type of players joined MLS this season (including Soony Saad), the most in MLS history.
Overall, definitely an article worth a look.
H/T to @presleylewis on twitter for bringing this article to my attention.