photo recovery after iPhone "Erase Data"
Which iPhone OS and model do you have? The newer ones encrypt the data and the erase deletes the key. The chances of DIY recovery are slim. If you pull it off, you probably can get hired for iPhone forensics and make some serious salary doing that professionally.

Older devices are more amenable to typical recovery efforts. Either way you will need a forensic tool like PhoneView to access the filesystem. Once mounted you could try something like Data Rescue. Neither tool was designed for this, so you should try the free versions first before spending money on them.

The professional data recovery houses will likely charge an arm and a leg to get your data back - but at least you can get a quote if you really need them back at all cost.

Be sure to double check your camera roll isn't backed up to iTunes or on Time Machine. It would be far simpler to restore the device to an old backup and get the photos off intact. You would lose any photos taken after the backup time, though.

If you have enabled data security (iOS 4.3 and iPhone 4 are one combination that enables this security), you're likely deep in the hurt locker. See iOS 4: Understanding data protection to tell if you enabled data protection. In a nutshell, that means the filesystem was encrypted and the erase destroyed the key to the encrypted files. You will need industrial grade forensics to get things back if the phone was powered off immediately after the erase. Using it after the erase will rapidly overwrite the scrambled data irreversably.