Hi there - this problem is caused by the latest update to the Play Store - fortunately there is method that seems to work.
First you need to remove the updates of Play store:
Go to Settings > Applications > Manage applications ... scroll down to Play Store and select it, and then press the Uninstall Updates button.
Once you have done this, reboot your phone.
After the reboot, open your App Market which will ask you if you want to download Play Store. Say yes to this.
Once the Play Store installation is completed - you should now be able to download Apps.
