Will you be looking to add a selection of lenses to your kit in the future? If so Micro 4/3 is probably the best option as it's a very well fleshed out system now.
If you are looking to stick to a single lens or two then your options are quite a bit wider, but Sony would probably be the pick of the bunch in that situation.
The crucial thing is to go and handle them, some of them are very very small so depending on your hands this may be a critical factor. There are also some fairly odd/unconventional form factors, so coming from a DSLR it's definitely worth trying them out to feel what is comfortable.
If you want to maintain a DSLR feel and have a viewfinder the Panasonic G3 is a bargain at the moment, the good thing with that is that there are a couple of nice fairly cheap primes (Panasonic 14mm 2.5, OLympus 45mm 1.8) should you want to investigate those in the future.