The Sox are out of the running. They don't really have any backup plans. The only major free agent I see Boston pursuing is Derek Lowe.
If the Angels can't get Tex, then they will most likely pursue Manny Ramirez or Adam Dunn. However, they are nearing a deal with Juan Rivera. (Props to MLBTR for the link). And an outfield that already has Gary Matthews Jr, Vlad Guerrerro, Reggie Willits, and Torii Hunter would be overcrowded with the addition of one of those two in addition to Juan Rivera. However, if Dunn or Manny would play 1st base, then both of them could be solid additions.
If the Nats miss out on Teixeira, then they are expected to pursue Adam Dunn and/or Orlando Hudson.
If Baltimore misses out on Teixeira, I don't see them looking into any other big moves that would benefit them for 09. I can see them trying to trade Brian Roberts and his salary before he reaches free agency after the season, and perhaps they'll trade Aubrey Huff, a free agent after this year as well, who had a great 08 campaign. If I were Andy MacPhail, I would be looking into free agent pitchers, such as Ben Sheets and Mark Prior. Both guys are great when healthy, but will be bargains because of their lack of recent health. The Brewers need pitchers, and if either of those guys starts dominating opposing hitters, a trade market will surely open up.
If the Yanks miss out on Tex, then we might see a push for Manny (I'm not a fan of this as we all know). But, if Cashman and Girardi, and not the Steinbrenner brothers, are in charge of free agent acquisitions this off-season, then don't expect a Manny signing. It seems to me like the Yankees will most likely explore the trade market for a center fielder and look into signing Adam Dunn or Orlando Hudson. They will most likely only sign Hudson if they trade Cano, or if Hudson agrees to play CF.