ok, after 10 days continuous and persistent and hectic searching (talking to apple as well) for a fix on the guest account after the lion upgrade, i finally made it work.
for some of us, only the user account was available in the login screen which means that we could not log in our user account accessing our data.
follow the steps:
1) remove the back of your computer
2) carefully remove the hard disk. (that is not difficult but make sure that you unscrew the tiny screws that hold the side metallic pegs too)
3) buy an external hard disk case SATA to USB, 2 1/2 inches. very cheap, bought mine for 12 euro.
4) connect your hard disk drive from your macbook or macbook pro to the external hd case you bought. by doing this, you transform the hd drive into a USb stick.
5) connect the external hd case to another computer. that will be read as a USB drive to the other machine so you can browse through all your files and folders.
6) go to: library/preferences/com.apple.loginwindow.plist
7) delete the file
8) remove the hd drive from the external hd case
9) put back the hd drive to your macbook
10) power it on and now you can see your user account plus another account named "other".
the trick was to remove the hd drive and use it as a usb drive.
this is the only way to remove the loginwindow.plist file from the machine.
all other ways through the safari only mode will NOT work as the accessibility the terminal gives you is fictitious, is NOT real.
hope you can all get your mac back.
ps this fix only applies to users who upgraded their lion to 10.7.2 from 10.7.0 and after reboot the only account showing in the login window was the Guest Account with the safari only mode available.