I'm debating the same question. In my mind, my images are split in three groups: model shoots, other 'serious' shoots and family/friends. Some of the model shoots involved nudity and in beta 4 I could use the 'Hide' command to hide those from my kids' views when working in LR. I don't see the 'Hide' command in LR1.0 anymore, so that's the first reason I was thinking of creating two libraries.
I also use a different name on my model shoots to split them from my other work, so in my test libraries the Identity Plate setups are different because of my 'split' personality.
Separating the family/friends would make sense purely because I don't intend to ever sell pictures of my friends/family and I do intend to market the other categories.
I'm still debating this though, because it becomes a hassle on importing when I've been somewhere with my family and have to split the 'serious' work from the snapshots by importing files to different libraries.
Btw, I do intend to use LR as my main image management system, but I will use Photo Mechanic to do the initial import and I will retain the original RAW files after I copy/import those as DNG files into LR.