1. Checking for pascal cased wiki words (as in WikiWord) and then either show them as links to a page existing (and formatting them nicely with a space as "Wiki Word") or making them into a link so a new page could be made.
2. Creating tiny rutines for either creating or updating on post back.
3. Oh - no step three as Mr. Jobs would have said ;-)

Looking into the source...
Let's see how a new wiki page is made including a security check to see whether or not our visitor has permissions to edit the page (warning: Source ahead):

// Validate user
umbraco.BasePages.BasePage bp = new umbraco.BasePages.BasePage();
bp.ensureContext();

// Create new umbraco document
umbraco.cms.businesslogic.web.Document d = umbraco.cms.businesslogic.web.Document.MakeNew(
Request["umbWikiCreate"],
new umbraco.cms.businesslogic.web.DocumentType(_wikiEntryNodeType),
bp.getUser(),
FindWikiRootDocument(pageID).Id);

// Publish changes
d.Publish(bp.getUser());
umbraco.library.PublishSingleNode(d.Id);

That's it. And using Web Matrix or Visual Studio .NET you even get IntelliSense to make coding even easier:

IntelliSense inside Visual Studio .NET gives you any information on umbraco business objects as you write your code!

And the code for updating is almost the same:

// Validate current user
umbraco.BasePages.BasePage bp = new umbraco.BasePages.BasePage();
bp.ensureContext();

// Get the current document from the business logic
umbraco.cms.businesslogic.web.Document d = new umbraco.cms.businesslogic.web.Document(_ph.pageID);

// Assign the document property to the current value from the textbox (the document property is based
// on the alias assigned through the variable "_wikiContentAlias".
d.getProperty(_wikiContentAlias).Value = TextBoxContent.Text;

// Publish the changes
d.Publish(bp.getUser());
umbraco.library.PublishSingleNode(d.Id);

// Redirect to the updated page
Response.Redirect(umbraco.library.NiceUrl(d.Id));
Response.End();

When I had finished writing my wiki, I made a couple of Public properties and some css classes for links to make it easier to customize. Now the css part makes it easy to format the different kinds of links (whether its new pages to be created or just links to existing pages), but the public properties can be read by umbraco and makes it possible for any user to modify settings in the wiki without a single line of code - just be dialogs automatically created be umbraco. All I needed to do was:

1. Copy the usercontrol and it's dll to my umbraco website.
2. Creating an umbraco macro which is showed below and click the auto-discover Public properties button
3. ...you knew it - heh, still no step three ;-)

Register the wiki in umbraco
To register my user control in umbraco it's necessary to create a macro. But it's a simple task, and just a couple of mouse clicks - look here:
Using my webbrowser I'm now inside the developer area of umbraco and by right clicking the Macro icon, I can create a new macro. So I'll be doing that and name it "Wiki" and press ok:

After that I'll edit my macro and tell it the location of my wiki control and when that is done, I get a button saying "Browse properties"... Let's try that:

Now there is the list of properties made in the usercontrol, and be checking the boxes, umbraco will create the properties inside our macro and make ui when the wiki is inserted. So let's try to insert the wiki macro into a template: