Short answer: no, it's not cheating.
A cheater is someone who breaks rules to get an unfair advantage or benefit.
In school, there are rules governing what students can submit on their assignments because it's unfair for someone who studied the material and worked hard to get the same grade as someone who merely copied an answer key. That's reasonable, given that the goal of school is to ensure that students are actually learning things and accumulating knowledge.
In the corporate world, the aim is generally to produce a high quality product as quickly and cheaply as possible, in support of the ultimate goal of maximizing profit. Employees are — or should be — encouraged to make use of all possible resources, like books, co-workers, mailing lists and online communities. There is no answer key to be copied or unfair advantage to be gained. (Ernest points out in the comments that this is not quite true, because of criminal activity, but I consider that to be outside the scope of this question.)
So, the long answer: using Stack Overflow at work is not and cannot be cheating, since the very concept of cheating has no meaning in this context.