There doesn’t need to be a generals election until 2010, however there is a belief, almost a superstition, in the Westminster village that calling an election in at the very end of a parliamentary term almost guarantees defeat.
I’m not sure if this is supported by data, and even if I is it could easily be a case of causation going the other way- Governments who are most likely to lose power (and know it) are also most likely to hang on until the very last moment in the hope that their political fortunes will change (see Major 1992-1997).