For the past 15 years, I've switched off from being an internal to an external consultant, and back again. I've learned a lot about how to be successful in each, and I've also learned a lot about the pitfalls of each. The best research I can recommend is Marguilles writing on the concept of "Marginality". That is, how much do you become "a part of the system" and how much you stay "on the outside". That's a really tough question.
What's most important is to consider the type of work you do, and how much of an external focus you need to hold to make your projects successful. For example, if you are an internal consultant, and you do Executive Coaching and 360 degree feedback, the issue of Marginality becomes critical. You not only need to remain a confidential and trusted source, you also have to be attentive to the "perception" of whether you are a trusted source. You'll need to be aware that quiet conversations in the hallways, can be misconstrued as gossip, even if you're just being considerate of your neighbors.
At the same time, if you mainly do software training, you'll want to be perceived as an "internal person" or "one of the guys". When you get a little mud on you, you will be more trusted, and therefore more successful.
