Further investigation found that any partition ending in a "10" was disabled from installation (64 bit version of LMDE). This was verified on 2011-04-16 when another install was attempted. It was noted at this time that in addition to SDA10 "greyed out", also SDB10 was "greyed out". This installer bug has been been submitted (hasn't been triaged as of this posting).
The final resolution (duh... ), I dropped sda10 and built a new ext4 partition (new SDA15). The installation proceeded good. The only caveat in all of this is that my /etc/fstab is now amiss for the mounted partitions after sda10 (renumbered).
____________________________End of 2011-04-17 Update_______________________________
Well, I finally decided to give the Mint Debian a tryout. At the partition selection during installation, the desired partition for installation was "greyed out". Specifically it is an ext4~8.9 G on sda. It formerly contained an old version of Mint Flux which was removed when I re-formatted the partition from ext3 to ext4. The other partitions containing my other ops are "active" in the partition selection at install.
Does anyone know what the criteria of eligible partitions for installation is?
Additional installation question on Mint Debian : during the install process will we be given the opportunity to select where GRUB is installed?