There are a couple features under development which will enable this facility in a decentralized fashion, for anyone using Bitcoin-Qt or bitcoind:
1) An optional address index is being added, for searches such as these
2) A pull request exists for "watch only address" support. A watch-only address is an address in your bitcoin wallet for which you do not have the private key. With this feature, bitcoind will dutifully watch for any transactions on the watched addresses, just like a normal bitcoin address you control.
Is this going to be available any time soon ?
I can't speak to the "watch only addresses", but I'm one of the developers working on an address index. It's a nontrivial undertaking, but finally in the implementation phase. The draft BIP and Python prototype implementation are about a week away from finishing (just need to implement and document some last minute changes).. although it's been "a week away" for the past three weeks as I work on other, higher priority tasks. It might take a little bit of time to be finished. The C++ implementation is only about 50% complete, and that needs to get finished before we can even think about finalizing the spec and deployment.
To really make efficient and secure use of an index requires a soft-fork, and it is less clear when that might occur. Look at how long it took for P2SH to happen, and that was a considerably simpler change originating from the core developers (I'd still count myself as an outsider). In the mean time though it might be usable as a merged-mined side chain.
