Interesting. The only thing which I can think of which will cause overhead here is that you need to enable all 1000 siteaccesses. This means that you will use some memory for these 1000 lines. But this overhead should be much smaller than having 1000 installations by far. Each site can share the same php code and this will be cached really well.
Haven't tested though. It would be interesting to hear about some test results ( mabye create a script which generates these siteaccesses for a test first? )
1.) catch all request on one ip, because we do not want apache to blow up.
2.) use mod perl to provide a fast mapping from external/unkown hosts to a common internal one
3.) do the usual mod_rw stuff 4.) prodive a little kernel hack to dynamicly match against a internal uri :-)