A 39

By default, verbal components need to be delivered in a strong voice. However, the game has several options which may alter this.
  • Sleight of Hand skill use (Races of Stone, page 133):
    Check: When casting a spell, you may make a Sleight of Hand check to make your verbal and somatic components less obtrusive, muttering magic words under your breath and making magic gestures within your sleeves.
  • Conceal Spellcasting skill trick (Complete Scoundrel, page 85)
  • Disguise Spell feat (Complete Adventurer, page 108)
It certainly looks like the Cheat spell's "fluff" text was written with that Sleight of Hand check in mind, as it's a very good correspondence in the RAW.