If rescue fails, Fobos-Grunt will reenter soon
The probe is currently stuck in a very low orbit measuring 207 x 339 kilometer after it's propulsion unit apparently failed, failing to lift it into a GTO (and from there an interplanetary trajectory):
If the probe isn't revived in due time - and the Russsian operators are still frantically trying to do so - it is doomed. With an orbit at this low an altitude, it is a short matter of time before it comes down again - another case of an imminent uncontrolled reentry of a very large satellite (over 13 metric tons, including the fuel). How much fuel is onboard is not clear to me: different media sources quote quite different amounts, but all amounts quoted are in terms of several tons, with several sources settling for 7 tons (see also here).
As pointed out here by Anatoly Zak, seizable chunks of the probe could survive reentry, and survival is certainly expected for the actual Fobos sample return capsule (which was designed for reentry).
Estimates of when Fobos-Grunt will come down are a bit complicated. Ted Molczan has noted that over the past day, the orbital evolution was unusual - Ted points out that if the last two orbit releases are not faulty (a possibility), it means that either the probe is manoeuvering (which from all the negative statements by the Russians in the press seems unlikely) or - more likely - has started to vent fuel since yesterday. As a result, it might have gotten a very mild orbital boost (the leaking fuel acts like a small rocket engine).
Before this unusual behaviour started, orbital elements 11314.14749491 to 11314.77184893 and Alan Pickup's SatEvo software with current solar activity levels suggested a nominal reentry time no later than early January 2012. SSC meanwhile predicts reentry for November 26th, 2011. The current unusual orbital evolution - if real - might change things a bit, but eventually it will come down.
This means it will likely come down somewhere over the next few weeks or months if the operators cannot revive it over the next two weeks. With an orbital inclination of 51.4 degrees, it can come down anywhere between 51 N and 51 S latitude.
There has been a call out to observers to observe the probe - it's brightness behaviour can yield clues as to whether it is starting to tumble, e.g. because of the suspected fuel venting. So far, observations by Brad Young and Michael Murphy from the US suggest the probe is stable in brightness with no sign of tumbling (see here and here).
Unfortunately, the probe is currently not visible from NW Europe where I am located: it makes passes near midnight, completely in shadow.