The most impressive enhancement in 2.6 (although, obviously this wasn't done exclusively for HA) is the improved robustness of the OS. It seems much less prone to emit the dreaded Oops (although when it does, it still erroneously tries to recover rather than doing fast failure).
The primary new availability feature is the proposed multi-path solution using the device mapper. Hopefully when this is implemented by the vendors it will lead to a single method of controlling and monitoring storage availability rather than the current 2.4 situation where each vendor rolls their own.
Finally, there are the indirect enhancements: those that improve Linux acceptance in the enterprise (where HA is often a requirement). Things like: