Target platforms are a mechanism in PDE that allows developers to identify the set of bundles and features that will be available to their application. You can then write bundles and compile against those in the target, define features/products based on target bundles, and run or export using those target bundles. Very powerful and convenient.
With the evolution of p2, you can define target platforms in terms of Software Sites. That is, you can point at p2 repositories and select particular versions of features to be used in your applications. Even more powerful and convenient. Now another step.
Starting with Eclipse 3.7M4 there is an Update button in the target management UI. Simply select the features and software sites you want to update and let PDE discover newer versions.
One of the coolest usage patterns this enables is targeting your builds. Simply setup your build system to publish its output into a p2 repository and then add that repo to your target and pick the relevant features. Then, whenever you want to pick up the latest build, click Update.