You talk of a penalty, where a search engine accessibility issue seems a lot more likely. SE accessibility issues usually translate to all search engines.
Besides that there are a few other possibilities that follow Occam's razor and don't promote the Bing-steals idea. The biggest of these would be trends or market changes. If all around the board you get less people searching for your product (for whatever reason), this should reflect in all search engines' stats.
It's also perfectly possible that an older niche player cleaned up his/her game, or a new competitor hit the scene. Both could eat up your traffic.
Or a webmaster that had many sites linking to you forgot to renew his/her domains.
I don't fault the OP for jumping to conclusions (well, maybe a little bit more care would be appreciated); search engines are a black box, and it is all too easy to connect changes you made on the site, to changes in stats. Even when they are not correlated.
Next time when you encounter a bump and ask yourself: Did I get a penalty? Really try to pinpoint the problem: Which parts of the site took a drop? Does that site still link to me? Is is a trend, and do last year's stats show the same decline? etc. etc.
http://www.seomoz.org/blog/whiteboard-friday-oh-i-got-a-pena... can help you with this. Also read up on how to submit a mobile sitemap and have these sites running next to each other without any (perceived) problems: http://www.google.com/support/webmasters/bin/answer.py?answe...