Are you referring to WS2003 Task Scheduler or some other scheduler within DNN? - sorry for the confusion, I am still learning about DNN internals.

With regard to memory overhead, doesn't it amount to much the same amount of memory whether you implement two sites (say) using a single DNN installation with 2 portals each running in a different application pool (2 processes) or two separate DNN installations, one for each site (2 processes)?

Taking on board your ealier point about the reasons for not using separate portals within a single DNN installation to implement separate sites, I did not follow what you meant when you said:

And the points made by Darrin McQuay and Chris Hammond regarding the need for a separate IP address for each site / portal if using SSL would presumably also apply to the case of two separate DNN installations, if each were running one of the two sites each with its own domain name?

Nevertheless, I find your earlier points regarding the general wisdom of using a separate installation for each site convincing and probably good advice, which I intend to follow.

My worries about disk-space are unfounded, I think, since I just looked at the size of my DNN installation on my VPS and, from the 4.8.2 install (without source) and with no extra modules installed and a single portal with no extra pages or skins it comes to about 23MB, so even after adding pages, modules, skins etc, I could probably fit a couple of hundred of these on my VPS.

Clearly, memory and availability of IP addresses are going to be much more significant limiting factors. My current provider only allows one IP address per VPS, so it looks like I'm going to be looking for another provider soon - any suggestions welcome.

Yes, I was referring to the DNN Scheduler which runs on a background thread.  It's application space will start it's own background thread.

Yes, two sites that are completely seperate would be roughly the same as having two application processes pointing at the same DNN install. 
My comment was in relation to one DNN install and someone saying that it took a lot of memory. 
Some people might expect that if you point two applications at the same install that there would be sharing involved and use less memory, but it really doesn't reduce the memory at all.

You are also right on in your finding that memory and IP addresses will be your limiting factors. 
Although the IP address problem may be solved with one of the new UNC/UCC SSL certificates.

http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/118/threadid/182168/scope/posts/Default.aspx