RE: 2.49b or earlier:
There’s a couple of ways, depending on whether or not you want to preserve the current cloth sim and all its frames.
Apply the Cloth modifier at whatever frame you want – this loses the other frames of cloth sim but is very straightforward and uncomplicated
Export the Cloth sim to a .mdd file (Vertex keyframe animation), then Import the .mdd data as shape keys (Load MDD to mesh RVKs) to a duplicate of your Cloth sim mesh, after removing the Cloth sim modifier. This makes shape keys of chosen frames from the cloth sim, which you can isolate to your chosen frame by a number of methods. The export/import process has a number of considerations concerning other modifiers, so it’s more complicated, but also more flexible in many ways.
Since soft-body is not a modifier on the stack, you may have to use method 2 for that kind of sim, though I haven’t tried it yet myself.