I had a nice combo while in SF at an Irish pub. It was a Cider with Guiness on top. They didn't mix (different densities - very cool, like an enormous B52). Tipping it back, the Guiness would stay on top, so you'd still be drinking both at once. Can't remember what the combo is called.

'round here, that's referred to as a "black velvet". here's the rundown on half/half combos:

black 'n tan: guiness and bass
half 'n half: guiness and harp
black velvet: guiness and cider
snakebite: harp and cider
bassbite: bass and cider

more folks seem to order black 'n tans, half 'n half, or snakebites than the others.

--dan.