We've received a number of reports that the Windows Phone Marketplace has been causing some issues at some point today, with the infamous "c101b001" error code when attempting to purchase (or download) an app. So far we're aware of trouble for the UK, U.S., Ireland, Netherlands and Italy.
The UK now has access again, so hopefully this is just a maintenance outage. Let us know in the comments should you be encountering problems, and which region you're accessing from. Some users have taken the issue to Microsoft Answers to seek assistance and populate a list of affected markets.
Thanks David (and everyone else) for the heads up!