I use a template when creating new ePubs - all the standard Cover/Title/CSS/OPF files already in a "default.epub" file which then gets Saved-As the new title. Everything has been going great until I realized that I've been forgetting to change the UUID in the OPF!

Is there some magic formula or scheme that should be followed when creating a new UUID?

And, more importantly, does Sigil have a function for doing that? If not, can it be added??
Depends on what kind of Unique ID you want to create. Random urn? ISBN? Your own custom scheme?

No (except when opening a new blank epub). The only way to get Sigil to generate a new urn:uuid (that I know of) is to completely remove the <dc:identifier></dc:identifier> entry from the opf file. Unfortunately, this also has the rather unfortunate side effect of Sigil believing your opf is broken--so it creates a new opf from scratch. Something you definitely don't want happening.
http://www.famkruithof.net/uuid/uuidgen might be of use.
@PeterT: Thanks! At least I now have 500 UUID's to hold me over!

@Diap: I was thinking a random ID... but the idea is to keep them unique. The link Peter posted uses a time stamp combined with a code based on his network gear?? I would think something along these lines would be very simple to incorporate into Sigil - a button in the metadata editor or something - especially since it already has the functionality to generate the code - we just need to tell it to do it.
Hi

There is a Linux standard command line tool called, what else?, uuidgen.
I simply use Jellby:YYYYMMDDx

where YYYYMMDD is the date of creation, and x is a letter I add if I create more than one a single day.
lol...I didn't realize it could be that simple. I thought it had to follow that 8-4-4-4-12 format.

Although, aren't you worried there might be another Jellby out there doing the same thing - thus making the ID non-unique??!!??
Out of curiosity, what would be the consequences if there were two separate epubs with the same identical UUID? Under what circumstances would the collision be detected?
I found it when I was using the firefox plugin ePubReader to read my books. When I added the second book (and the third, fourth, fifth etc.) with a UUID of a book that was already in the catalog, it would delete the first book and replace it with the new one...thinking it was an updated version of the same book.

I haven't had that issue with the limited number of other readers I've tried, but it seems like it could cause a problem.
It ought to be reported as a bug, so that the developer can throw a dialog box in there asking if it is the same and the other should be deleted. It may only be deleted from its own internal index, but still....