bit-pay is introducing interesting and unique solution, so check with him
there are bitcoin online wallets that allow or may allow easier acceptance of bitcoin payments on websites such as
www.instawallet.org &
www.mybitcoin.com (some users have reported some problems with this service after hack of major bitcoin exchange, mtgox)
additionally, exchanges offer or may offer easier acceptance of bitcoins through their API service, check with mtgox.com api page and tradehill.com api's as well
I'm thinking about looking into php integration solution sometime in the near future although no promises on this one at the moment.
disclaimer: i can't vouch for any sites and services listed above, i just mentioned what i've learned and heard about. I have used mtgox exchange for purchasing some BTCs direct on their site. their APIs are used widely for reporting and tracking trades, executing trade orders by traders and their bots, and should be suitable for merchant's sites as well.