There isn't full instructions yet, and apparently the method of doing so breaks a bunch of OTHER stuff on your 1.0.2 iPhone, so I wouldn't advise doing it quite yet, but a hacker named digitalgecko has successfully run the MobileStore.app on a 1.0.2 phone!
"WARNING: Currently Following Any Instuctions To Enable the Store Will Break Almost Everything Else, You WILL be Restoring Your Phone
digitalgecko has successfully run the MobileStore.app on a 1.0.2 phone. Doing so requires replacing alot of frameworks so its not currently recommended. The store functionality is limited as itunes will not configure a 1.0.2 Iphone for Mobile access."
iPhone Dev Wiki