I have a similar issue only I cannot get to the grey log on screen when I enter my Mac user name. I am set up on Lion as a user without a password. If I try and connect using PocketCloud and enter my Mac admin user name and password it does indeed log me in. This is no use to me though as the admin account is used only for updates and system management.
I have tried various other things when prompted for my Mac Credentials such as leaving my user name blank and only adding the VNC password,adding my user name and the admin password and of course just leaving the password blank.
This has only been an issue with Lion since I updated PocketCloud today. On my iPhone it is still not updated and it let's me enter the VNC password then I get my Mac log on screen and I can click my user name and in it goes.
Any ideas or help?