Narayanan,
Please take a look at the PMBOK Guide 4th Edition, section 11.5.2 on page 303. This question comes straight from here:
A fallback plan can be developed for implementation if the selected strategy turns out not to be fully effective or if an accepted risk occurs.
A fallback plan is also part of the risk register:
Fallback plans for use as a reaction to a risk that has occurred and the primary response proves to be inadequate;
The important distinction to keep in mind here is that a fallback plan is not a workaround. A workaround is "A response to a negative risk that has occurred", while a fallback plan is developed and implemented if your primary risk response strategy turns out to be ineffective.