The CSS validator is based on CSSTidy. It comes with some exclusion rules to prevent false positives and you can add your own. I use a couple of CSS hacks/filters, namely “*property” for all IE and “_property” for IE6/5. These regular expressions will prevent the validator from reporting errors related to the hacks. Enter them one at a time at Preferences -> Aptana -> Editors -> CSS -> Validation:

.*unrecognized : \*[-a-z]*: [%\(\)-_'"./?0-9a-zA-Z ]*; for .*
.*unrecognized : \*zoo 1; for .*
.*unrecognized : : for null.*
.*Property _[-a-z]* doesn't exist.*
.*Parse error - Unrecognized .*