Now they are aware of the fact the rules can be interpreted differently, it will not take their engineers of any of the teams long to figure out how to build a similar diffuser. These are after all top of the bill engineers.
Problem is in the architecture of the rear end of the cars, especially the gearbox and rear axles. Will they allow the new diffuser to be fit with minor effort, or do major modifications need to be made. If not the latter, most teams should have a race-worthy version ready for use in China (not Force India). They won't use it before anyway, as the WMSC could still ban these diffusers, but fabrication should already have started at some teams.
If major changes are necessary, they will have them ready for the race in Spain surely. Red Bull (not STR, who have a completely different drive-train) seem to have a problem with their gearbox, such that they'd have to re-design that as well. This won't be ready in 3 weeks, but it is something they can test on the bench.
Testing is of course banned until January 2010. Teams can only run straight line tests, or tests with young drivers without real F1 experience. These won't help test the diffuser's functionality.
Of course, certain teams might also want to improve the reliability of their cars. "To finish first, you first have to finish," as the cliché states.