There are a few reports in a Bing Forum thread that adding a site to Bing Webmaster Tools might not work. Specifically, if the URL or domain contains a hyphen (dash) such as www.best-domain.com.
Brett Yount from the Bing Webmaster team confirmed the bug, saying:
Currently, we are having a few difficulties which I just received confirmation from the indexing team. They are currently working on it, but said that if you try a couple times, it should work. If not, and your site isn't in the index at all , please post on the not in index thread and I will work to get your home page (only) into the index.
I personally tried adding a domain with a hyphen and it worked for me on the first try. So maybe it is resolved or maybe those specific domains have other issues?
Forum discussion at Bing Forum.

Comments:
Brian
12/17/2009 03:44 pm
I submitted my site to Bing Webmaster Tools over 4 weeks ago and it still only has 1 page indexed. Compared to Google Webmaster Tools with over 70 pages indexed. And I have no hypen in my domain. Very frustrating
Rene
12/18/2009 03:48 am
I think the Bing webmaster tools is still under testing phase. But it helps if you've already placed your website there for tracking and indexing on Bing's search engine.
Van
01/03/2010 05:00 pm
I thought I was the only one having this issue. From the indexing point of view I think that bing is very accurate in its rankings however it can take a little longer than usual.