A single sign-on is VERY desirable - but personally I have no idea how to achieve this.
It can be done, but you need to pick your "home base", for example the forums.
Then you have to modify the code in the other apps, like Wiki or WordPress.. change the code that uses their own registration to read the tables from the forums.
Alternately, create an entirely new userbase table(s) and modify any app you use to use those new tables.
We did the former and then migrated to the latter for a group of sites I run that run multiple instances of PHPbb and FlashChat all using a single, unified userbase.
don't ask me for specifics tho, as I'm not the guy that did the code work.