BitPing.Net (BPN) is a service that enables merchants to accept bitcoins with a minimum of effort.
Users add a list of public addresses to be monitored, and BPN sends a notification when a payment is received.
Notifications can currently be made using Email and HTTP POST
To use BPN in a webshop, you need to generate a "large" amount of addresses.
Then upload these addresses to BPN, and select the number of confirmations you require (1-6)
When a customer selects to pay using bitcoin, you select a bitcoin address and assign it to the order.
If the user pays, BPN will send a confirmation email/HTTP POST(once it has been confirmed as many times as requested), which you can use to mark the order as payed.
You can use Vanitygen to generate the addresses, then insert them into your database, and export the addresses to BPN (One address per line, no seperators).
The code for the monitor is opensource, so you can run your own server instead of using BPN if you like.