Yup, I think yun ang ibig sabihin ni smasher02, hindi ka makakagamit ng sim ng ibang network. Not sure kung meron nang unlocker for iPhone 4 pero it is possible na maging open line yan pag na release na yung unlocker.
Sa jailbreak naman don't worry sa updates, kung meron sobrang important na update you can restore your iPhone 4 para matanggal yung pagka jailbreak, then you can update (just remember to back up your files).
@smasher02
Hindi ako sure kung pareho sa US pero I think mas mura yung sa MObinil na iPhone 4 dahil kailangan may contract ka sa cellphone service nila for 2 years and usually network lock yung iPhone, so sim lang nila pwede mo gamitin. Siguro yung international version na sinasabi nila is yung factory unlocked, no need for contract, kahit anong sim pwede (basta micro sim) kaya mas mahal.