I generally I agree with the comments about the dynohub if you're constantly commuting and need light. I own 3 bikes and 2 of them have dynamo hubs on them (though I'm regretting not buying a lighter wheel for my latest one - I got a cheaper $150 wheel, and now I kinda wish I had gone all out and gotten a light $300 wheel).
But there are some drawbacks to dynamo wheels and light -
1. There's still *some* drag. It's really not a lot. I cannot tell while riding whether the dynamo is on or off, and with the dynamo off my wheel still spins a rather long time if I spin it by hand and just let it spin to a rest. But on really long trips (did 70 miles in 2 days) I got this feeling like turning it off made things a tiny bit smoother somehow.
2. As you can see from what I mentioned, they can be expensive (since you usually already have a wheel you're replacing). A really nice and lightweight one costs $300. You can pay $400 for the top end SON hub built into a lightweight wheel, or $150 at the lower end (like I just did). And you haven't bought a light yet, which costs between $100 and $200 (the Cyo, at $100, is nearly as good as the $200 ones though, to be fair, so say $100 for the front light, $25 for the rear, and another $25 in wiring - but that's assuming you install it yourself).
3. An Ultegra hub is 200 grams. The lightest Shimano dynohub is supposedly 490 grams. I'm not 100% sure, but they also may not come in anything less than a 32 spoke version. So it will add a certain amount of weight to your front wheel.
4. IMO, dynamo lights put out enough light to bike by, but not necessarily enough light to completely comfortably bike by. For example, my Cyo tends to put out enough light on the road, but not much to the sides - city riding where you can kinda see everything is great, as is biking in the moonlight. Biking through the woods where I feel like something might jump out at me means I bring my wider beam battery light as well. To be fair though, none of my battery lights light up the road as nicely and evenly as the Cyo.
There are some other advantages:
1. Already mentioned was no time spent charging or changing batteries.
2. As already mentioned, you turn the front light on (or just leave it on all the time and it turns on when you start moving) and the back light comes on automatically.
2. As they have limited input, dynamo light tends to treat their light very preciously and have a nicer controlled beam which doesn't waste light, as I mentioned previously with the Lumotec Cyo.
3. It's easier to "permanently" attach your dynamo light to your bike. They're generally screwed on, so no one is going to casually walk by and walk off with it. If you're really into securing them, you can find ways to make it difficult to get off the bike even with tools (like putting solder in the screw heads). Since you don't need to change batteries you don't need to take them off the bike on a regular basis. They also are as tempting to steal (IMO) because it's useless without a hub, and most people assume they're crappy lights.
So they have their pluses and minuses. I use battery lights on my "race" bike, but that's also made possible by the fact that I *never* leave in unattended outside because it's to expensive.
Since I'm typing, there are 2 lights I know of (wish there were more) that, while they don't control the rear light or anything, are less of a hassle for charging and stuff:
1. The Ixon IQ - same optics as the Cyo I mentioned above, though unfortunately less light output (40 vs 60 lux). Not much light off to the sides. I personally wouldn't recommend it as I don't think it puts out enough light by itself (many many other people disagree with me, but that was my feeling).
http://www.peterwhitecycles.com/b&m.asp
2. The Light and Motion Vega 200:
http://www.bikelights.com/info.asp?uid=229&p=13
Both these lights are self contained, and yet don't require changing batteries - you just plug the charger into the light and the light charges the batteries. The Ixon IQ lets you replace the AA batteries yourself, the Vega 200 has non-replaceable batteries. Still, less work than changing out batteries.