Just reviewed the Retraining Rules in the PHB2, and noticed the rules for Skill Retraining are pretty lax:

Subtract up to 4 skill ranks from one skill and add an equal number of ranks to any one other skill (not including Speak Language). The skill to which you add the ranks must be a class skill for one of your character’s classes, including a class he is about to gain with his current level increase. It doesn’t matter whether the lost ranks were purchased as class skills or as cross-class skills.
This seems like it provides a loophole that can really help me in my current situation of trying to keep Tumble maxed on my Rogue/Swashbuckler. On Swashbuckler levels I can put four skill points into a Swashbuckler class skill, then next level retrain those four into Tumble at the same cost. With a little bit of patience and forethought, you get a lot more flexibility than the class skill list intended, almost eliminating the need for Able Learner.

Then there's the obvious technique (specifically suggested in the book itself) of using retraining to move ranks out of skills that are no longer needed -- such as hide after you get invisibility, heal after you get magic items, or listen/spot after you get Mindsight.

What other clever applications of retraining are there? The other two I came up with recently are:

1. retrain Precocious Apprentice after you get level two spells
2. retrain Dodge to Expeditious Dodge after you get more speed (such as with Scout or Swiftblade)