It took me weeks to pin down, but if you're having Lion Firewall problems which include not allowing Remoter into your Mac, then you could try this:
delete /Library/Preferences/com.apple.alf.plist
reboot
--> the Firewall will be OFF.
turn OFF and then ON the sharing services you wish to make available, including in this instance Screen Sharing.
turn ON the Firewall.
if desired, click the Advanced button to access the advanced Firewall options such as stealth.
From this point on, (on my Mac Pro running Lion 10.7.2), the Firewall runs as expected... **until** the Mac is rebooted again. Something in the reboot process apparently corrupts the pref file.
cheers.
(VNC Remoter 3.2.1 on an iPad v1)