With Windows 2008 R2 and Windows 7, you can allow for alpha numeric characters for your operating system drives. The Group Policies in Windows Server 2008 R2 have expanded from the R1 version. In the R2 version, you have the option of configuring BitLocker for fixed drives, operating system drives, and removable data drives.
Expand Operating System Drives and open Allow enhanced PINs for startup. Setting this policy will allow your BitLocker PINs to have both upper and lower case letters, symbols, and spaces.