Eight speed will be around for a while and most eight speed components can be replaced with nine speed parts without trouble. Cranks, hubs and derailleurs are completely interchangeable. Nine speed shifters should shift eight speed cassettes (not using one gear of course). Chains are backwardly compatible (nine speed chains work perfectly on eight speed drivertrains.)

I haven't tried it, but I suspect that eight speed shifters will even work on a nine speed cassette sans one gear. In any case, if you're worried, buy a spare cassette or two and you'll be fine for many years, probably until nine speed is gone. Even then, ten speed also seems mostly compatible.