I'm ignorant of this. Have an example to share?
As one example:
PHB Stealth:
+5 Trained
+3 feat Skill Focus
+2 power Elven Boots (once per encounter)
+1 to +6 item Elven Cloak or +2 Ring of Invisibility
So, the PHB was limited to +14 (+16 sometimes, which is still very high in a D20 system).
+2 background (background skill boosts did not exist with just the PHB)
+5 unnamed Rat Form Armor (once per day, but still)
+2 to +6 item Boots of Stealth, or +5 Lightstep Slippers, +3 to +6 Shadowform Armor, +1 to +6 Sylvan Armor, etc., etc., etc. The advantage here is that the PC is not forced to use Elven Boots and Cloak, but can mix and match and get other item selection options.
Trickster's Mask (2 rolls instead of 1, ~equal to +3, plus once per day auto-20)
Chameleon Ring (no need for cover or concealment once per day)
+2 power Battlestandard of Shadow
+2 racial Group Stealth
+5 feat Stalking Panther form (druid only)
+5 feat Camouflage (ranger only)
unnamed Secure Encampment (+wis mod during extended rest)
This does not include powers or paragon path/epic destiny abilities. For example, Panther Ancestors can eventually be as high as +8 or even +10 unnamed. Or the number of ways Invisibility can be added now. Or feats like One With Shadow. Plus there are a lot of ways now to acquire concealment.
Also, I only looked in Martial Power, Adventure's Vault, and PHB II. I'm sure there are more elsewhere. Without even cracking open another book, I remember that familiars often give a +2 unnamed bonus to some skills.
And this is only one skill. Granted, some of this stuff is conditional and some of it is not just item based, but the fact remains that people can take these and boost their Stealth (or many other skills) pretty darn high. Higher than what the PHB alone allowed.
Where there only used to be a few ways to acquire a decent stealth skill in the PHB, now there are many ways and it can be higher than just with PHB rules. It's also not just the final bonus to the skill, but all of the associated synergies as well.