Using a standard action skill in combat usually provokes an Attack of Opportunity. If you incur any damage from the AoO, the skill attempt normally fails. There's a Concentration check exception to avoid failure when damaged from an AoO (mainly used when casting a spell) with DC = 10 + damage dealt to you. You can also use a skill defensively with a DC 15 Concentration check to avoid the AoO in the first place, but if you don't make the required DC 15 check then the skill check automatically fails and you waste your standard action.
There's also a feat (Cutpurse, from Dragon # 322) that lets you use Sleight of Hand in combat without provoking Attacks of Opportunity.
One final thing to remember is that while filching a dagger is a DC 20 Sleight of Hand check, trying to pull a dagger out of an enemy's sheath ups the DC to 50. (DC 50 applies to any size sheathed weapon.)
There's also a feat (Cutpurse, from Dragon # 322) that lets you use Sleight of Hand in combat without provoking Attacks of Opportunity.
One final thing to remember is that while filching a dagger is a DC 20 Sleight of Hand check, trying to pull a dagger out of an enemy's sheath ups the DC to 50. (DC 50 applies to any size sheathed weapon.)
