Leah's right in that you *don't* need citizenship to stay in the UK- getting Indefinite Leave to Remain (after 2 years as a spouse) means that you can live in the UK for as long as you like. However, it is a new requirement to take the Life in the UK test to qualify for ILR- just came in last week, in fact.
When does your visa expire? If your FLR expires after April 2007, you are required to take the test and should take it after 2 April 2007 (since that's when they're coming out with a new version of the test...I'm not sure whether it's okay to pass the old version and use that for ILR next April.) If you are eligible to apply for ILR before 2 April 2007, then you won't need to take the test and can apply for ILR under the old rules.