I personally prefer one dnn instance for each domain.   In my very first installs back in 1.0 I put them all in the same instance which at first worked fine and made things easy to upgrade etc.   As I started adding domains managing it got more and more complicated.  Upgradng became a real issue.   Some of the domains had 3rd party modules that I had to wait for releases from the software vendors while othere were pretty much core modules so they could have been upgraded.   I finally broke them out earlier this year and am very glad that I did.   It is more of a head ache to upgrade to newer versions but I don't have to wait for all 3rd party modules to be updated in order to upgade the other portals.  Plus if I do something stupid and break something I only have 1 site down where as in your case you would most likely have all 80 down if you did them as one instance.   I hope this helps.

Thanks for the input.  It looks like each method has it's advantages and drawbacks.  See if I have this right.

Domains all under one instance:
Advantages:

• Less risk in updating
• Problems created by one site does not cripple all sites

Disadvantages:

• Core upgrades have to be done on every installation
• Module installation must be done on every installation
• Cannot share content from a shared host since there isn't one

Sub-Domains under one instance:
Advantages:

• One Upgrade for all
• One module installation for all
• Can add content from the host  level

Disadvantages

• Bad upgrade blows everybody up
• Problems in one site can affect all others

So it really seems to come down to convenience of centralized updates and module installations versus risk of one failure point. Does that sound like the gist of the dilema?  AM I missing anything?

Thanks