Hello marnan and Welcome to Mrs. Clean's Forum.
I did a bit of research on your issue of cleaning the slats of your hardwoods. The information I found said that you just vacuum and/or use a soft brush (as you are doing now) and that solves the problem! There is no further information about stubborn dirt.
Now for the good news.
I did find that there are specific wood floor steam cleaners available and that they seem to be very effective on loosening stubborn dirt. Please follow the link to find out more:
http://wiki.answers.com/Q/Can_you_use_a ... ood_floorsAlways be sure to check with your manufacturer before using any products or methods so you don't void your warranty. I have requested information from a site and will let you know if I have anything further on this.
Keep me updated and let me know what you think on the steam cleaners!
Good Luck,
Mrs. Clean