Likely scenario, either;

They'll be dug in deep, with a complex bunker system, around and within the city itself.
Fighting fanatically to the last. Mostly die-hard Saddamites with nothing to lose. Stiffened with a large influx of foreign radicals who will stand in the face of an Abrahms with an RPG.
The air strikes to date will pale in comparison with what will occur if this is true. The future of Takrit as a city might be questionable. If you have watched closely you may have noted the extent of Saddam's military engineers ability to build an impressive underground infrastructure. ( yeah I know, bunker busters. But, also I have to point out, there have been many underground structures untouched by air strikes. )

Or, if we, and the civilians in Takrit are lucky. They fold up like most of the rest have, esp in Mozul and Kirkuk. The occassional skirmish with small groups of lunatics of course. But no serious combat.

CentCom is prepared for the first, and hoping for the last. Like I am.