The rules say a ranger's combat style feats apply only when he wears light or no armor. This rule seems a bit unwieldy in that the player and DM will have to keep track of when the character got a feat.
If a ranger wearing chain mail takes Point Blank Shot as his standard second-level feat, he can use it whenever he wants, but if he takes it as a combat style feat, he can't use it when wearing his armor. That seems overly cumbersome to me.
I can't think of any other instances in the rules where someone with a feat is limited on when he or she can use that feat, outside of limitations in the feat description itself.
Apologies in advance if this issue has already been covered elsewhere. I didn't see it.
The Ranger's combat style is simply a class feature that has the same mechanics as a pre-existing feat; it is not actually a bonus feat. If the designers had intended for the Rangers to actually have the appropriate feats they would have simply given it to them as bonus feats. You may also notice that Rangers don't have to qualify for said feats in order to count as having them. The fighting styles represent special class specific training that makes Rangers capable of wielding two weapons or firing two arrows in a single round in light armor. The only reason it mentions the feats is because said feats are pre-existing game mechanics that accomplish the same effect. The limitations are in place because they are not actually feats, but are class features.
From the Beta:
"Combat Style Feat (Ex): At 2nd level, a ranger must select
one of two combat styles to pursue: archery or two-weapon
combat. This choice affects the character’s class features
but does not restrict his selection of feats gained through
normal advancement. He can choose these feats, even if he
does not have the normal prerequisites."
He has them, but only in light armor:
"The benefits of the ranger’s chosen style feats apply only
when he wears light or no armor. He loses all benefits of his
combat style feats when wearing medium or heavy armor."
Ahhh, but they are, in operation, feats. If you take "Precise Shot" as a CS feat, you then qualify for "Improved Precise Shot" as a normal feat. The downfall is that if you wear Heavy Armor, then you lose both feats ("Precise Shot" due to the armor, "Improved" since you no longer meet the Prerequisites).
IIRC, you can take any feat you want at any time. You only gain the benefits of the feat if you meet the prerequisites. At least that's how I always interpreted it. It's also how you were able to get some Epic level feats that required an Ability score in the high 20's, which is almost impossible without magical aid.