At last week's Senate Judiciary subcommittee hearing, advocates of closing Guantanamo, such as chairman Dick Durbin and Human Rights First president Elisa Massimino, talked about how to close Guantanamo: in particular, by transferring or releasing most detainees to other countries and then moving the remainder into the United States. Of those moved into the United States, many would be prosecuted (whether in civilian or military courts), but an undetermined number of very dangerous detainees would continue to be held without criminal trial under law-of-war authority until cessation of hostilities – that is, until the end of the ongoing war against al Qaida and its close allies. Some version of this seems to me to be the only realistic approach to closing Guantanamo.
Although pulling this off will require that President Obama spend tremendous political capital, it would actually push some very difficult decisions onto his successor's shoulders, too.
Even if it closes Guantanamo along the lines laid out above, it's very unlikely that the Obama administration will have prosecuted or found alternative security solutions abroad for some number of the most dangerous detainees (by most credible estimates, at least a few dozen). It's also very unlikely that they'll simply release them – especially because whatever political deal Obama strikes to close Guantanamo will probably include assurances that he won't do that.