Some time ago before APC changed servers, someone ran a thread asking for opinion on alternative fuels in transport category aircraft. Here are two articles that seem to inidicate biofuel/ aternative fuels are likely to happen.
"Boeing may consider GTF for 737."
Flight International (9/17, Kingsley-Jones) reported, "As Airbus prepares to flight test Pratt & Whitney's GTF geared turbofan demonstrator with the view to a possible re-engined A320 development, Boeing has hinted that it would not rule out a similar move with the 737." According to Flight International, "such a program would be relatively straightforward, given the installation of the A320's current powerplants and its relatively tall gear, despite the GTF's larger fan diameter." And while "the 737's shorter gear makes a GTF re-engining more problematic...Boeing is quick to point out that its engineers say that it would not be impossible." For example, "one option the company has examined to enable the GTF to fit under the 737's wing would be to slightly tilt the nacelle up at the fan case."