From what I've seen, by far the biggest factor in whether or not a person in IT or in any other department for that matter keeps their job during the 'merger' shakeout depends on where they came from - if they worked for the bigger/richer company, their odds are a hell of a lot better than if they worked for the company being acquired.

When the company I worked for 'merged' with a bigger/richer one, I went all out to find a position elsewhere and within 2 months, found something with more responsibilities/learning opportunities and more money; within 2 months of my leaving, 2 of my colleagues who were in comparable positions to mine had done the same; within 3 months of that, more than half the our remaining colleagues including our boss, peers and lower level techs had been let go. About a year after that, hte majority of the remainder of our IT organization had been outsourced...

So, based on my experience, get out early if you are from the smaller/less profitable company - no matter how good your reputation or accomplishments have been there, it isn't going to be your current management making the decisions.