Yes there is - its called multisites - there is a mindfield of information out there as to how to get this to work - but it does work. The problem with single login is that you need to be sharing the same database for each site so that you can specify the pnTables to share users and session and varmodules tables
(anybody else please add the other tables that i have forgotten) In addition you will need to edit pnSessions so that the domain name that the cookie recognises is the same for each site. - This too you can find by searching the forums here.
I think multisites are amazing - once i had it working right - but the problem is when you start to get rather large databases for each site.
Aparently if you are a mysql whiz there is a way to share tables from different databases but you need to have access to the mysql settings.
I know that was rambled - buit do a search for
multisites and
multisites session and also read the
Wiki support pages in support.postnuke.com