Chris, I've personally worked on sites using three different paradigms:
1. Subdomain: something.example.com
2. Variant New Domain: www.somethingexample.com
3. Subdirectory: www.example.com/something
From the perspective of search engine optimization, numbers 2 or 3 might work best in the long run. Number 2, only because the link text if others link over to it will be more likely to reflect the "something" keyword, which helps build relevancy for that term. But, #2 can take quite a while to gain traction as the "sandbox effect" can make it take a while to establish a new domain and get it ranking.
Number 3 can be preferable for fastest SEO traction, since you'd be founding the new section pages on an existing, already-established domain name.
Trinorthlighting is right, though -- if you're just delivering up the same identical content as on your B2C site, the B2B site is going to just result in duplication, which can wanter down your PageRank per page. Doing that could reduce your rankings on various keyword SERPs, which could reduce your traffic. So, be sure to offer up substantially different content on that new section, or else set up the new section under robots.txt to not be spidered.