A quick analysis of the situation

The situation is evolving so rapidly that I hesitate to put thoughts down. Still, here's my take on what's happening:

1. Although we still don't have details about what powers Mubarak has transferred to Suleiman, it's become pretty evident that Suleiman is in charge. 
2. Why then keep Mubarak around? Aside from the loyalty the regime's key men have for Mubarak — Suleiman, Tantawi and Shafiq have 20 years of being close confidantes to him — retaining Mubarak allows them to preserve the sanctity of constitutional authority.
3. Who cares about the constitution? Perhaps not many protestors, but for the regime the constitution represents legitimacy. Mubarak needs to be in place, even if only symbolically, for amendments to the constitution to be made. If the constitution is suspended, then this forces the army to take charge itself (presumably through the Supreme Military Council), which opens the way to demands for civilian government and lifts the last layer of distance that the army has vis-a-vis the people.
4. Why wouldn't the army want to take charge directly? Because it makes it directly accountable to popular demands and opens the way for calls for a new civilian transition government that could challenge or dilute its own authority. A civilian government that could for instance instigate wide-reaching corruption investigations.
5. The army could also be split on this issue, with hour-by-hour negotiations taking place between those who back the protestors' demands and the senior officers. It may also want to avoid an armed clash with the Republican Guards that would seriously destabilize the country and further rob the regime of legitimacy.
6. Have we passed the point that the army is becoming a target of the protestors too? There's always been a core of activists who want to see the end of military dominance over Egypt. It's not clear whether it's the majority, or even if this sentiment is echoed in the wider, silent Egyptian public. The army's key problem (and especially Suleiman's) is that they suck at communicating. Their battle to retain public legitimacy may be lost because of bad PR and tone-deafness.
7. What about the US in all this? The Obama administration has made a good step in returning to an emphasis on transition and the protection of civilians. However, its handling of this crisis has been poor, its statements mealy-mouthed and at times contradictory (esp. between State and the White House). It has shown it has neither control nor particularly good information on developments. This crisis has also revealed one of Obama's core problems, in domestic and foreign affairs: lack of resolve and initiative, to put it kindly. But there's no reason to dwell on this: for now the US is largely irrelevant to the events on the ground. It should keep quiet, back democratic transition, cut off military aid until this is achieved, and start rethinking (as the EU is starting to) its support for dictatorships in the region.