Yeah, this question comes up in a lot of root forums for both Amon_RA and ClockworkMod recovery.
Also, in case you want to know, the name format of the Nandroid backups that Amon_RA creates, BDS-ccyymmdd-hhmm, tells you that:
1. The boot (B), data (D), and system (S) partitions were included in the backup. There are lots of other things that the nandroid-mobile.sh script could backup and cause this part of the directory prefix to be a little different: C = cache, E = ext2, M = misc, R = recovery.
2. The date the backup was created is ccyymmdd
3. The time the backup was created (GMT, I believe) is hhmm
Also, as the guys indicated, there are certain character that "play nice" with the nandroid-mobile.sh script and those that do not. What actually determines this is how the nandroid-mobile.sh is invoked by the custom recovery. Its done via a shell script launch (/sbin/sh) and the nandroid-mobile.sh command with all of its parameters, including the backup or restore directory name, is passed in the command that is passed to the sh shell that is launched. So, using characters that might be interpreted other-than-intended in the backup directory name, might cause the sh shell to become "confused" or just not work right.
I added into my "Verify Nandroid Backup" function in my AFV app, a built-in test/check to analyze the name of the backup directory that the nandroid.md5 file is in to look for certain, "sketchy" characters that might cause problems. The ones I added include space, asterisk, question mark, greater than, less than, vertical bar (pipe), semi-colon, and amperand since I could envision circumstances in which the nandroid-mobile.sh command would certainly not be properly interpreted by the shell should these characters be used.
Hope that helps. Cheers!