Aluminum isn't exactly toxic or harmful to the same degree as lead, but it's not exactly good for you either. And as you've identified, aluminum is fairly reactive. Higher acidity, salinity, and cooking time will all contribute to further reaction and absorption in any aluminum cooking vessel (or utensils for that matter).
This is an issue for three primary reasons:
- Absorption of aluminum is, as noted above, not necessarily healthy.
- Absorption of aluminum can result in a metallic flavor in your food, which is generally unpleasant.
- Absorption of aluminum into your food also corrodes the cooking vessel.
That third is probably of the most concern here, since you're talking about a vessel that's under pressure when in use. Sufficient corrosion and the resulting structural weakness could (in very rare cases) cause a rupture and sudden pressure release, i.e. hot stew explosion. This would take a long while to occur, and you'd almost certainly notice the corrosion on the inside of your vessel before it became truly dangerous, but this type of problem is exactly why modern pressure cookers have such fancy pressure-sensitive locks and come with warnings all over them. They can pop if abused, and they do this in your kitchen, possibly while you're nearby.
So, extended cooking of certain acidic items (what if you wanted to make, say, tomato sauce in your pressure cooker?) could be dangerous with long-term use, hence the warning. Such foods will also contain more aluminum than they would otherwise, and taste like it as a result. Here's a thread with information on the general pH levels of common foods, which may help you identify specific items of concern.
Another good way to avoid this is to select a stainless steel model instead of aluminum. Stainless is more expensive and doesn't heat up as quickly, but it's not as reactive as aluminum either, so there's less concern when cooking acidic ingredients over a long period. Here is a decent buying guide with comparison of features and some more specific recommendations.