Troubleshooting MacBook Air boot issues
If single user mode boot fails, the OS is broken on a fundamental level and since you already did the reinstallation of Snow Leopard the mac now should boot into safe mode or single user mode if it were working properly.

No matter how bad your user account or third party software was corrupted, a safe boot after a reinstallation of the OS should work. It's odd the installation didn't flag an error since i've almost never seen an install complete and not be able to get a safe boot/single user mode boot afterwards. 

You did a great job of following the steps in Mac OS X: Gray screen appears during startup - I'm assuming you checked the peripherals, etc...

It's probably time to make a call to Apple for service, but if you care to try some more - here are two things I would do.


fire up disk utility and repartition the drive - make three partitions to see it can accept a new partition map, then make it back to one.
write zeroes to the entire drive


It's rare, but sometimes the drive needs to be really cleaned out and installing into the existing partition clearly didn't help you. Both of these make it almost impossible for even a professional to do data recovery if you don't have a backup - so use care before trying these.