Shimano 105 5600 derailleur compatability with 105 5700 11-28 casette My current cassette is a 105 5600 12-27 the front is a 105 5600 53/39. I want to go compact so will the Shimano 105 5600 rear derailleur support the 105 5700 11-28 cassette?
As long as the 5600 and 5700 cassettes contained the same number of gears, the two cassettes will be interchangeable. They're both 10-speed, so you'll be fine. You'd have had problems going from a 9- to 10-speed, or going from a 10- to 11-speed. The reason in each case would be because of chain width.

update - as @jimirings points out, the RD-5600 is spec'd to only support cogs with up to 27 teeth, so it is possible there may be problems, although I'd have thought it unlikely. (In fact I too am riding a "28" cassette with a "27" derailleur and have not seen any ill effects.) If you're really concerned about this, possible workarounds would be using a different variant of the CS-5700, or upgrading to an RD-5700. If you're willing to spend to upgrade your crankset, the additional cost of a rear derailleur is quite small.

Incidentally, if you're wanting to go compact, why are you changing to rear mech? I mean, its no problem if you want to do this, just as long as you realise you don't need to do this. 

To go compact, you can just replace your current crankset with a FC-5750, or even with an FC-5650 if you can find one. Try looking on eBay, you may find a bargain.