A runestaff (MIC p. 223) allows a character who's attuned to it to sacrifice one of their spell slots to cast a spell of equal or lower level from the staff. This allows characters to cast spells that they don't know, though they must still be spells from their class spell list. From MIC p. 224:
By expending a prepared arcane spell or arcane spell slot, the wielder can cast a spell of the same level or lower from the runestaff’s list, as long as that spell also appears on the wielder’s class spell list.
Per my previous question on this topic, it's possible for a multiclassed spellcaster to use a runestaff to cast a spell that's on the spell list of Spellcasting Class A, but using a spell slot from Spellcasting Class B. For instance, a multiclassed Duskblade/Wizard using a Runestaff of Enchantment could use expend a 1st level Duskblade spell slot to cast Sleep, a spell that appears on the Sorcerer/Wizard spell list, but not on the Duskblade spell list.
Normally, a spell cast via a runestaff uses the character's caster level. Again from MIC p. 224:
The spell is treated exactly as if the wielder cast the spell herself, including caster level, save DC, and any other effects related to the spell.
However, in the case of the Duskblade/Wizard above, the character has two different caster levels. When casting Sleep from a Runestaff of Enchantment, does this character use their Duskblade caster level, or their Wizard caster level?