Firstly, you can now purchase and browse apps from a desktop PC.
From the Windows Mobile Developer Blog:
There are also improvements to the developer's portal and stronger anti-piracy protection.
Starting today, Windows phone customers can browse, buy and download applications online at the Windows Marketplace for Mobile site. The selected applications are delivered wirelessly to the customer’s Windows phone and install the next time the Windows Marketplace client runs on the device. This creates another way for customers to easily find and purchase applications and gives developers a whole new level of exposure.
Secondly, the actual app has had another update, I guess to implement some of the new security features. Only thing is, it has stopped all of the [ahem], preview versions from working! All is not lost though, as there is also an updated v1.1.1109.000 file to get your preview version up and running again.
Lastly, the official 6.0 and 6.1 updates are