conditional partitioning at install time

[Leonardo Cuyar Morales <leonardo@softel.cu>]

I'm making a custome debian squeeze installer iso, with unattended 
settings for locales, language, packages install and custome postinstall 
scripts, all of this in the preseed file. Is posible a configuration or 
a script to make posible to maintan a previous partition if it exists 
(without afecting the mount point and size) or create a new one with the 
same moint point and size if not exists?