In my continuing quest to understand the effect of head angle on handling I've been looking at what happens when you add in a larger wheel.
Initially I thought the reason you wanted a slacker HA for going downhill was so that you didn't go OTB when it got steep. If that were the case then it should apply equally to 26 and 29 inch wheels. Also the difference is only a couple of degrees which is pretty small compared with the difference in steepness of various parts of a trail.
Now, I'm tending to think that it has more to do with keeping the front wheel on line as you hit bumps. The caster effect of a slacker HA achieves this, but so does a larger wheel. Hence a steeper HA is required on a 29er, otherwise it never turns.
I notice that GaryLake mentioned taking the Orange Gyro (with its 69 degree HA) down the FOD downhill course. So that would suggest that a 69 degree 29er is as capable as a slacker 26" bike downhill. But then we have the Five29; a 29er with a 66 degree HA, which kind of ruins my theory (unless it rides like a 64 degree 26" bike or something).