Lightroom 1.3's exploitation of the Camera Raw Cache is very new and I haven't had time to experiment or reflect on it, so for now I'll just point to this thread Lightroom 1.3 creates CameraRaw Cache and quote a couple of posts from Thomas Knoll:

It speeds reopening files in the develop module. It speeds rebuilding the library previews after a settings change.

You are not seeing any benefit to the cache because is too small by default and is “thrashing” in with your usage pattern. If you have the disk space to spare, increasing the cache size to 10GB or more would help a lot. The goal is be able to fit cache file all the files your are “actively working on” into the cache. For example, if you import and process about 500 photos in a typlical shoot, you really want the cache to be able to hold at least 500 most recently accessed photos for the cache to provide significant benefit.

For Camera Raw, it uses stores 1024 pixel (long side) images in the cache, so the cache will hold about 200 or so images, which for most users is smaller than the average shoot. Lightroom 1.3 is storing the “standard size” image in the cache, which by default 1440 pixels on the long side (and can be higher--set the catalog settings dialog), so far fewer can fit in the default 1GB cache size, so Lightroom is more likely to thrash than Camera Raw with the default parameters.