Hmmm seems like the docs need to be updated - most still say bridged mode is unsupported...
From 6.4.2.4 release notes:
AP Fast Failover Support for Bridge-mode Virtual AP High Availability (HA) support for bridge mode in Campus AP is introduced in ArubaOS 6.4.1.0. In previous versions of ArubaOS the fast failover feature for Campus AP was supported using tunnel or decrypt mode. Now support has been extended to bridge mode as well.