Getting FAST Search for SharePoint 2010 installed is a pretty involved process.  It’s definitely easier in the RTM since there is a wizard now and you don’t have to run 20 different PowerShell scripts.  However, one issue I encountered myself a few months ago was an error I received in the Configuration Wizard.  It occurs on the screen prompting you for the name of your FAST User account.  When you specify the username and password, you might get the following error.

User Validation Failed.  Username or domain is not valid.

Immediately, you assume that you typed the password in wrong, but you soon realize that is not the case.  I honestly don’t remember where I found this information otherwise I would give him or her credit.  I’ve seen a few people run into the issue in the forums lately, so I thought I would post something on it.  To resolve the issue, you need to grant the Allow log on locally privilege to your FAST user account in your Local Security Policy.  Add it and run the configuration wizard again and you should be up and running.