Interesting question..
Let's look at what our fearless leader is doing. She has a highly successful Forum and Internet Marketing infomation portal at
www.SelfStartersWeeklyTips.Com and a highly read, rapidly growing successful blog at
www.ClickNewz.com.
I rather doubt Lynn's success would have been any greater if it was
www.SelfStartersWeeklyTips.com/blog, I actually would be of the opinion her results might be less.
I think your real issue is this. Are the blogs purpose to promote that associated site, and that is its only purpose? If so, make it a subdirectory of that site.
If the blogs are intended to have standalone power, and an identity unto themselves, then leave them separate.
The other advantage to having them under separate domain names is the fact that they will be EXTERNAL links coming into your other sites, instead of internal links.
Just some food for thought..