I'm one of those IAs that began their career in project management. I was good at taking a vague customer aspiration, a distant goal and a pot of money and turning that into a realistic plan with workable time and resource allocations constructed using a sensible methodology, with set UCD deliverables, points of review, contingencies and dependencies.
My heart was never really in it though. After creating a pretty Project plan, I kind of lost interest in cracking the whip, and all the minutia of LLPs, status reports and the rest. Plus, I was always more interested in the quality of the solution, not whether the project delivered on time. Hence I was not a very good project manager, despite once telling Rob Love, the now ex-CEO of Victoria Real, that I could do project management with my eyes closed. But then again, lots of people talked crap at VR...
Planning user-centred design activity
When it comes to planning user-centred design activity, what is key is that the activity is actually planned and is not left to chance or circumstance.
At the same time, a usability plan should not be distinct from a larger project plan. Instead, it should be intrinsically entwined throughout that larger plan.
User-centred design activity should, in fact, be built right into the way the project is structured and run. And needless to say, the best way to do that is in the context of an agile methodology rather than its stepped linear waterfall cousin.