I'm 6'2", all legs, and spent 2009 obsessed with rider position. After much experimentation, I am currently running a custom 420mm seatpost and a 30 degree riser stem on a Large SIR 9 SS (suspended), and am finally happy with the fit and ride. My bars are about 3/4" lower than the saddle (which I rider very high). If the bars are even or higher than the saddle, I don't get enough weight on the front wheel for sharp handling. Any lower than 1" and I find everything (in trail riding) suffers. YMMV.
With no bike-fitting experience, I became lost (and broke!) in endless combinations of stem length, stem rise, h-bar width, h-bar sweep, h-bar height, seatpost height, seatpost/saddle offset, saddle angle...and even fork sag! Plus my bike has an EBB, the position of which affects almost every other measurement on the bike simultaneously.
I still can't get over what a difference 10mm of stem length makes, or an extra 5mm of headset spacers. Its intersting and educational to surpass the correct fit of a part, go ride, and then bring it back to the "sweet spot." Realizing how many possible "configurations" are possible, I decided to document my different set-ups, and am currently on SS configuration #26 (no, I'm not joking). And yes, this is a great web site:
http://alex.phred.org/stemchart/Default.aspx
I agree with Wish I Were Riding (post #15) that finding an efficient position for standing climbs on a single speed is key. I agree you must get your torso errect, and for me, its key that your arms are almost locked out, and not too far in front of you - this creates a rigid upper-body (in lieu of a saddle). Total h-bar height is the obvious adjustment here, but stem length, h-bar width, and frame size also factor in. With the upper body errect and the arms close to your body and nearly locked out, you maximize your skeletal structure and upper body muscles for awesome climbing power/efficiency (like at the top of a dead lift). Of course, this only works if your body is balanced front to back on the bike so your front tire stays planted while the rear wheels gets lots of weight. But even then, its not over - you have to take your dialed-in standing climbing position and harmonize it with your bike's overall handling. With so many things able to be adjusted, one can go crazy. And nobody else's fit matters much b/c your body is unique, not to mention your trails, riding style, flexibility, and bike frame.
For me, I'm pretty fond of bars being at or slightly below the (XC) saddle height, as well as wider bars and shorter stems (especially on a single speed), whether for climbing, handling, or descending.
Long stems make absolutely no sense to me, whether the h-bar is run even with the bars, inches lower, or between. Even on a road bike, I find long stems and low bars just leads to decreased power, and a sore neck and knees. Then again, I'm pretty inflexible, and don't need an aerodynamic position for wind-cheating race speeds.
One other thought: I think going from a 26er to a 29er may require a very different fit, depending on the 26er and 29er, and how you ride each. Many 29ers have steep headtubes and seattubes, and chainstays around 18". Meanwhile, 26" bikes have slacker angles and chainstays under 17". It seems to me that long-stay 29ers are inherently more forward biased than 26" rigs, and so saddle-to-bar ratios may need to be re-thought.