Seriously? People are claiming the ability to cast spells isn't an ability? It's listed under "Special Attacks" in monster entries. Special attacks are a subset of special abilities. Plus, you know, it's an ability pretty much by definition, since everything a character can do (even breathing air) counts as an ability (though breathing air is, of course, Na, as it's not a special ability). I can't believe people are now trying to argue that "Spells" isn't a special ability. Open up the Monster Manual, find the first monster with the ability to cast spells, look where it's written. Now look at the next monster. Now the next one. Now the next one. Now do the same for MMII. And III. And just in case anyone's not clear, the phrase "extraordinary special attacks" comes up in numerous places (such as Rules of the Game: All About Polymorph, which has the most designer discussion on the topic of ability types).
Plus, we've got people claiming that only Na abilities are unlabeled and spells must be Na... go look at the PHB. Check the Fighter. See "Bonus Feats?" It's an ability. It's not labeled. But feats are always Ex. Now look at all the other classes, and note how class abilities that aren't magical are almost always unlabeled... but since they're Special Abilities, they can't be natural (also, check out the multiple definitions of Natural which all state without exception that they're stuff inherent to the form of the creature... NOT things you learn). So, that leaves Ex.
If you look through the books, you will in fact find that virtually every ability that's not labeled, but is in fact listed as a special ability or special attack, is in fact Ex. Improved Grab? Ex. Sneak Attack? Ex. Bonus Feats? Ex. Spells? Well, there's a kinda obvious pattern...
But just for anyone not clear on what Na abilities are, here's some definitions:
Rules of the Game:
Natural Ability: This term is a catch-all for just about anything a creature can do (or characteristic that it has) that is not extraordinary, spell-like, or supernatural. Natural abilities include most speed ratings (some very high speeds are not "natural," see the section on the alter self spell), mode of breathing (lungs, gills), natural armor and weaponry, general appearance, body type, and the presence or absence of the five basic senses (sight, hearing, touch, taste, pain). When polymorphing, you generally lose your own natural abilities and gain those of your assumed form.
PHB Glossary:
A nonmagical capability, such as walking, swimming (for aquatic creatures), and flight (for winged creatures.
PHB 180:
Natural Abilities: This category includes abilities a creature has because of its physical nature, such as a bird's ability to fly. Natural abilities are those not otherwise designated as extraordinary, supernatural, or spell-like
As you can see, it's not stuff you learn to do. It's stuff based on what you physically are. A Solar's fly speed is Na because it has wings. Its ability to cast spells? Not so much.
JaronK