They were able to compensate for mediocre relief pitching in 2007 (94-68) and 2009 (97-65) because they scored a lot of runs, 822 in 2007 (#6 in ML) and 883 in 2009 (#2). That formula probably won't work anymore because they finished 19th in runs in 2010 and are currently 19th in 2011, so the Angels better relief pitching if they're going to challenge the Rangers.
Will it continue? They don't have as many solid veterans as they used to or an all-star closer, but Jordan Walden (0.00 ERA in 7.1 IP) and Rich Thompson (1.59, 5.2) have a lot of potential. Scott Downs (first season with Angels) is capable of having a great season and Fernando Rodney might be ready to finally break out.
With a rotation that is strong at the top (Dan Haren, Jered Weaver, Ervin Santana), they can stay competitive if their relief pitching is in the top 10 and they get a little more offense.