Speaking mandarin in Beijing?

Not having much (or any) Chinese language in Beijing isn't quite as bad as you'd think. 

Many companies for which you might end up teaching have opportunities for their English teachers to also take Chinese lessons. They will also often do much of the inital leg work for you in regards to your visa, bank account, and housing (or they may simply send a Chinese speaker with you when you go). When negotiating with a school, make sure they will help you with these things, because they are the most difficult to figure out on your own.

As far as everyday life goes, there is a large expat community (particularly on the east side of the city), and even though most locals don't speak any English, you can do an amazing amount with a little creativity and a lot of trail and error. To get an idea of what I'm talking about, I'd highly suggest you read the Newcomer's Guide that I wrote for people in just such a situation. Basically, if you're living in a few areas that are "foreign-friendly," you can get away with just knowing some easy-to-learn phrases that will allow you to do pretty much what you need to do.

Living in Beijing as an expat is definitely an experience worth having. I hope you do decide to come and check it out. Good luck!

There are two ways. First one will take many years but if your ambition has nothing to do in being a China scholar, the second way will allow you to learn alphabetized Mandarin in a year or so. True, the second one is strictly for conversation only.