Last night, Bing has updated their webmaster guidelines adding a section about "keyword stuffing." Surprised it wasn't there from the onset? Yea, me too but truthfully, there are a ton of things they can/should add there that are not currently there.
What is new? The section on keyword stuffing, which reads:
When creating content, make sure to create your content for real users and readers, not to entice search engines to rank your content better. Stuffing your content with specific keywords with the sole intent of artificially inflating the probability of ranking for specific search terms is in violation of our guidelines and can lead to demotion or even the delisting of your website from our search results.
I verified using various caching services that the paragraph was indeed not there a day or two ago.
That being said, the language is pretty strong. If you do use keyword stuffing techniques on your site, Bing may give your site a "demotion" or even worse "delist" your site from the Bing search results.
Hat tip to +GauravGupta2014 for informing me about this.