I'm rather late to this discussion, but I wanted to confirm and reiterate a few points that have already been brought up, as well as add a bit.
For reference, I work at a small fuel cell research lab in the US, so I have some idea what I'm talking about. There is one running on a test bench about 8 feet behind my head as I write this, and 3 more stacks capable of producing ~1KW each within sight of where I'm sitting.
That said, the fuel cells that will go into laptops are unlikely to be the same type as the ones that will go into cars or power plants (which are the type I work with). They are more likely to be something more along the lines of a hydrogen battery--still a closed process, but that can be run in reverse to "charge" it off of electric current (plugging your PB into the wall).
There would be no external waste, and no fuel added--the theoretical advantage being more stored power.
You could also run a fuel cell off of stored hydrogen cartridges, or a hydorcarbon like methanol or maybe butane. Hydrogen could be stored in a hydride, which would prevent any major leakage, stores a lot of hydrogen at low pressure, and in fact gets colder as you suck hydrogen out of it, so it could actually be used as active cooling.
In the case of hydrocarbon fuels, you would drop in a little fuel cartridge, which would make for uber-fast recharges, nearly limitless carryable power storage, but a distribution problem--you'd probably want an internal battery as an alternative, maybe something you could swap out.
In any of the above cases, the fuel cell is not likely to run very hot, probably no more so than a battery being charged now, and maybe cooler. The amount of water produced would be very small, and would be in a vapor form; I assume it would be stored internally (in the fuel cartridge as it emptied), or it could just be exhausted out the back as a tiny warm, moist airstream--it'd be less moisture produced than when you exhale, so it wouldn't exactly be dripping out the back.
One thing is for sure: these are on the way, and they're going to happen. As pointed Toshiba, as well as NEC and several other companies are well along in the development process, and I'd look for refined versions in a few years max. This is not second page news, it's the cutting edge.
Lastly, a couple general notes about hydrogen as a fuel: It isn't a fuel, it's an energy storage medium (some people here understand that, many don't seem to). You make it with something, then react it with oxygen to make water and electricity. You can extract hydrogen from a carbon (usually fossil) fuel, or you can make it from water by putting energy in (more than you get back out later, but that's the cost of energy storage). Ideally, you'll be getting hydrogen electrolyzed from water using renewable energy, in which case the environmental impact would be minimal.
Hydrogen is actually a reasonably safe fuel, and the FAA has already started to make concessions for carrying hydrogen powered laptops on planes. Likewise, as pointed out, the Hindenburg didn't explode because of the hydrogen, it was a rocket fuel coating on its skin. The burning hydrogen after the explosion went straight up, not injuring anybody.
And the ozone issue brought up by that Cal Tech study is based on accurate science, but highly questionable in pratice--the leak rate they assumed is nearly 10 times what would be realistic, and a simple catalyst on the vent would prevent ANY hydrogen from leaving a system.
You'll be seeing major consumer use of hydrogen within 20 years, and laptops may well be at the forefront much sooner. I hope Apple is front and center.
Argh, there goes my credibility.