The N900 does not have a 1 GHz processor. It appears a fair number of N900s happily accept being overclocked that far, but not all do. Nokia can't release an official firmware update that overclocks devices to make Flash 10.1 run when doing so will crash devices.

Yes, OS2008 did bump the processor in N800s above the normal clock speed for that OMAP SoC, but in that case, Nokia had paid TI all along for speed binned parts, tested and certified to be stable at the higher speed. That's not true of the N900.