I've asked this before and was told it was impossible with PHP for various reasons that I can't fully remember.
If you want to view the posts from around the 8/9/10th of July you'd find it, but I haven't got the time on a 56k modem 🙂
Its annoying, maybe PHP5 will contain it because I know I'd find it useful, you can always grab as many details as you can and try to match them up if your trying to use it to block / identify duplicate members or something similar.