A notice on Battle.Net's European web site says that Blizzard recently became aware of an issue in the digital installation downloader for Diablo III's English and German versions. This affects European versions only, not North American.
They're asking that anyone who downloaded the game before Saturday (today) at 10 a.m. Central European time/9 a.m. GMT, to go get the new downloaders now (available at the link below). Doing so will ensure they they can play the game when it unlocks on the May 15 launch.
There are specific instructions and more details at the link. Spread the word so no one gets left out and is disappointed when the big day gets here.
New Downloaders for English and German [Battle.Net. Thanks to DeviantVanguard for spreading the word.]