Is it normal for a fixed gear bicycle's rear wheel to slip forward in its dropouts from time to time? My fixed gear bike has a track fork with a chain tensioner, and uses Pitlock's solid axle nuts to secure the wheel in the dropout, though I had similar issues with the stock nuts.

After tightening the nuts with a torque wrench to the 260-390 inch-pounds recommended by Park Tools, the wheel appears secure, and will typically be fine for weeks at a time.

Inevitably however, the rear wheel slips forward on the drive side, perhaps by 0.5-1cm. This produces considerable slack in the chain, though it's usually still ridable in the short term.

My guess is that this is happening under sudden braking, which I've heard can cause slippage when the nuts are insufficiently tight. Indeed, I usually notice it after having to stop sharply somewhere along the ride. 

Is this normal, or an indication that the torque specification I'm following is incorrect? If normal, is this preventable with some maintenance (for instance, is it caused by gradual slippage?). If it's not normal, what's the cause? Are there perhaps different torque specs for a fixed gear bike?
This can be a fairly common occurrence with a fixed wheel bike. It may depend on a few different things, ie what sort of nuts you are using, how tight they are, what style of dropouts, and what the dropouts are made of.
A different sort of nuts may help. eg something with serrated nuts or washers could grip better. Also you may be able to tighten the nuts more, maybe a bit higher than what Park recommend.
Probably the best option is to use chaintugs. These fit on the dropouts, and have some sort of screw to hold them in place. So this stops the axle from moving forward. Usually you only need one, on the drive-side dropout, though you can use one on each side if you want.
One example is the Surly Tuggnut.

(source: surlybikes.com)