I''m happily auditioning Amethyst 1.5 RC2 with Flex 4.5 (flex_sdk_4.5.0.20967) with no real problems as of yet.
There are only a few things I've noticed that reflect the fact that the feature list [1] doesn't yet say "Supports Flex 4.5". Specifically, 1) a scarcity in Flex 4.5 specific templates and 2) the new spark DataGrid is not available to the designer.
Do you have an ETA for "official" Flex 4.5 support?
[1]
http://www.sapphiresteel.com/spip.php?page=amethyst-feature-list