Using a Wiki Page for Blog Comments
I think wikis can replace blog comments right away. Most blog comments are text-only so use of wiki text formating rules and WikiWords is optional. When a comment about a post is received either directly from blog tools/services or indirectly via a spider, monitor, or another blogger, receiving script can create or find a wiki page to add the comment and the original post to the page.
One great benefit is that comments are no longer second class information: isolated, unindexed, and often overlooked.
Wiki to Trackback and Congregate
The script can also walk through links mentioned in the comment or original post to discover related blog posts and drop a comment as breadcrumb for other bloggers to follow. This functionality is like trackback but with added benefit of congregating bloggers and commenters to a shared space.
There is the possibility of multiple wiki page being created for a single 'conversation', but most of them should be mergeable automatically. Still, more thought is needed here.
Blogs within Wikis and Pageroll
In wikis I have seen, some users create a personal wiki page mapped to the user's name as WikiWord, where users can add personal information there. There is also a way to retrieve a wiki page containing the history of a user's activity within the wiki.
I think these pages can be redesigned to resemble a blog with its own set of RSS feeds. User activity history can become a category within that blog. Auto-population of blogroll is also possible here with names of people the user interacted with. In this context, wiki pageroll listing wiki pages the user frequents makes sense.
My brain is still humming with thoughts on ways to combine blogs with wikis. Between the two technologies, Wiki is the fragile one so I am trying to take extra care not to kill the humming bird. Stay tuned.