There are 2 great articles that will help you get the most out of your Web-to-Lead conversions.

Associating Campaigns with Inbound Leads notes that you can auto-associate a lead with a campaign by adding the following fields:

<input type="HIDDEN" name="Campaign_ID" value="7013000000004ko">
<input type="HIDDEN" name="member_status" value="responded"> 

You can have i-Dialogue automatically associate a Lead with a Campaign if the Web-to-Lead form is on a landing page associated with a campaign. To create a campaign landing page, drag a web page onto a campaign object in the i-Dialogue tree menu.

A best practice tip is to create a campaign named something like "Google AdWords - Keywords" and then associate a landing page containing a Web-to-Lead form with the campaign. Not only will the Leads in Salesforce be automatically credited as responding to the Campaign, but the Google total impression count and click-throughs will be updated as well.

Passing Hidden Values through Web-to-Lead Forms builds on the advanced feature of using hidden fields to pass any type of information from the web server to a Lead record.

Watch this HOWTO video to learn how simple it is to create a Web-to-Lead form in i-Dialogue (There's no editing tricks. It really does take just a couple minutes to publish a lead form in your portal).

Creating hidden fields in i-Dialogue is easy. Add a question to any form of type "Textbox" then in edit mode, select the "Question Is Hidden" checkbox. That's it! Salesforce recognizes the Question Code to be the field ID and Default Answer to be the value.