I apologize if this is well known, but I certainly didn't know about it. I was reading a build on The Skyrim Blog and it detailed a way to block while dual wielding. http://theskyrimblog.ning.com/group/character-building/forum/topics/character-build-the-swordmaster is the build.
Anyway, it is done by having a single one-handed weapon in your right hand and blocking with it. While blocking open your menu and equip another one-handed weapon to your left hand and close the menu while still holding the block button. Your character will be blocking with the left weapon and holding the right weapon above them and if you block bash you'll do a swipe with your left weapon. This makes me think like they were originally going to implement dual wield blocking because there are animations for it.
Here are some pictures:
http://cloud-4.steampowered.com/ugc/577890220547300742/95ACEF63D072EF399EBE19AC49C6B394A1903D6D/
http://cloud-3.steampowered.com/ugc/577890220547302030/BAD5F61A5FA99904C552F0775EF7BB2BFE1C9ACF/
http://cloud-3.steampowered.com/ugc/577890220547303266/16593FEEAAF95C61FF9BDEB4CA24B9B1C6E31B18/