I am working on a solution based on bitcoind and Abe with some PHP/MySQL added on.
Its not as lightweight as the bitcoinnotify, but I am sure someone (or myself) will also offer it as a webservice for those whom does want to run bitcoind+Abe locally.
bitcoind downloads the blockchain, it only contains the key it makes during first start.
Abe populates a MySQL database from the block chain.
My script checks the database for any new transactions to the monitored addresses, and if any are found, with the needed number of confirmations, a notice is sent by whatever medium is needed (ie. email, pubnub, http post, database update, sms, etc).
The system is modular, you can replace any part as long as the interface stays the same.
ie. bitcoind can be replaced for another client that saves blockchains in the same format.
Abe can be replaced by another script that maintains the database schema.
My script can ofcause be replaced/modified as needed.
In contrast to bitcoinnotify my script will be open source, and so it bitcoind and Abe, so anyone with a server will be able to run it for their own good, or develop a website around it to provide a similar service.
The security is comparable to the notify system, you only store the public keys on the server, so any compromised server can only be exploited to send fake reports about payments, they cannot actually use the BTC stored on the addresses.