XDA Developers released this Android Marketplace app. Download and install the APK. You will then be able to browse and download apps from Android marketplace.
This apk has been reported to work for several phones such as stock g2x, sgs2 2.3.4 , HTC Thunderbolt running CM7, evo running miui, and many others.
Try it out!
Download: [Update: vending.apk has been taken offline since many users reported errors after installation.]