On a scale of 1-10, where 10 is that you're about to go over the handlebars unexpectedly, it's about a 6. Seriously, a visible crack in the rim radiating from the spoke means the wheel is about to fail. It probably won't be catastrophic, but it will wobble badly and you won't be able to true it. Carry a spoke wrench. I had a wheel fail like this, and I managed to nurse it home after messing with the spokes a bit on the side of the road- otherwise the brake rubbed even when released completely.

Go buy a Mavic Open-pro laced to an Ultegra hub with butted spokes, for about 125 bux at Performance. Then take it to a good shop to be re-tensioned. They're bulletproof wheels when tensioned correctly. Or, get a new rim that has eyelets with the same ERD as your aerohead, or add spoke washers yourself, and rebuild your wheel. I just plain wouldn't use a rim without eyelets.