I'm eating a lot of kale and other leafy vegetables lately, and I'm concerned about removing pesticide residue. Is thoroughly rinsing the leaves in cold water enough?
When washing vegetables that can get muddy (Kale, leeks, etc.) I generally recommend first using tepid to slightly warm water to help soften the mud more easily. Leeks should be split down the middle and then can be fanned under warm water which will help wash the dirt out more readily.
For Kale, if it isn't muddy then you can use cold water and give it a soak and agitate as hobodave indicated below. If they're muddy and dirty, then I'd use slightly warmer water to first clean them and then give them a soak in some cold water to help plump them up and increase their turgor pressure to make them nice and crisp.
A good wash in water (universal solvent) should be enough. Most of what I've read on the effectiveness of "vegetable wash sprays" say that they're no better than a good wash with water.
Also, make sure that you cut off entire stem right up to the top off the leaf, otherwise the consistency of the finished leaf will be a little too chewy...
Leafy greens like kale should be washed in a sink or tub full of cold water. Submerge the leaves in there and agitate them gently. You do not have to worry about pesticides. Your greater concern should be with removing all the sand and dirt from these leaves. Chewing on a rock, no matter how small is not a pleasant experience for you or your guests.