I travel by bike wherever I go whatever the occasion. Don't have panniers, but a good backpack does the trick. When going with BF, he carries the drinks, I take care of the food. His backpack fits a 12pack of beer perfectly (or a couple of bottles of wine depending on the occasion). For food, I usually bake any or all of the following:
*bread or buns
*cinnamon rolls/raisin buns - always a popular dessert
*lefse (Norwegian flat-bread type things filled with cinnamon and sugar)
*Pound cake baked in a bread pan. Put it back in the bread pan when cooled and pack in a ziplock bag. Bring the glaze separately in a jar and pour over cake when you get there.
*salad - just get a big plastic tup with a secure lid and it will travel just fine (saves you the trouble of tossing before you go...). Bring the dressing separately and dress on arrival.

All has been well recieved on many occasions, especially the baked goods. I usually find that people forget about desserts for a potluck. Very disappointing. I'm a dessert person and will quite happily skip the salads and veggies and things that everyone else thinks to bring.

Mmmm....I'm hungry now....if only there was cake around.....