First, some half remembered info.
My current understanding is that 4th edition will have three types of encounters. Combat encounters, Social encounters, and Skill Challenges. The Skill Challenges (for lack of a better name, also, I cannot recall what such encounters were referred to as) would be the means by which a DM would put the players in peril with rock slides, a forest fire, or confront them with things like a locked door.
Skills will work mostly like Saga, with a minimum base rating for the skill, which is probably similar to Bab, and a trained rating, which would add +X. The logic is that if you throw an obstacle like a loose rock wall that the party needs to climb, it is not really ideal if only 1 or 2 people in the party are able to climb it. Also, since various Skill Challenges could show up in combat (think of a sword fight on a narrow beam over a lava pit, where you need to make skill checks to stay on the beam), it is hard to put a level appropriate Skill obstacle into a fight when you have one guy with max ranks in the skill.
If I recall correctly, the gap between an trained check and an untrained check will be +5 for the trained guy. Significant, but if we assume that a typical skill DC will be scaled so that after adding the untrained / character level bonus, a typical challenge needs an untrained character to roll a 10, and a hard skill check calls for a 15, the +5 bonus for being trained can be significant.
Now for speculation:
I suspect that for skills which can be used untrained, or generally obtained as a cross class skill in 3rd edition, everyone will have a reasonable chance at overcoming a level appropriate obstacle. This would probably cover Climb, Swim, Balance, Ride, Spot, Listen, Bluff, Sense Motive, Mundane Knowledge skills like History, Craft, Profession, Hide, and Move Silently. I also suspect that some classes will automatically be trained in some skills (Rogues would get Spot, Listen, Hide, Move Silently, Wizards would get Knowledge, etc). Some races will probably have a flat bonus to some skills (Elves and Move Silently, Dwarves with Craft). Some skills will be class limited / trained only (Disable Device, Pick Pocket, Spell Craft, Forgery). And some classes will simply allow for more skills to be fully trained (allowing for Rogue and Ranger to retain their edge over fighters when dealing with skill checks).
I suspect that if someone really wanted to be a hard core skill monkey type, they will be able to build such a character by using Feats to be considered Trained in more skills, thereby getting a bonus to those skills.
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