Got this from the Salesforce.com knowledge base! Essentially you'd have to have a different web-to-lead "jump page" for each campaign you'd be running...

Here are the instructions:

If the URL to your campaign in salesforce is like the following:

https://na1.salesforce.com/camp/campaigndetail.jsp?id=7013000000004ko

You will need this part of the campaign URL: 7013000000004ko in order to associate leads captured via web-to-lead pages online.

To setup Web to Lead Capture:

1. Click on Setup | Studio | Customize | Leads | Web-to-Lead
2. Follow Steps 1-3 to configure your Web to Lead setup.

Click on the generate HTML option and ensure you select the 'Campaign' & 'Campaign Member Status' fields to ensure they are included in your HTML.

The following code will need to be included in the HTML of the web-to-lead page in order to associate the leads submitted with the campaign:

<input type="HIDDEN" name="Campaign_ID" value="7013000000004ko
"></select><br>

<input type="HIDDEN" name="member_status" value="responded">

This code will hardcode that webform with your campaign using hidden fields that the user will not see on the webform.

When someone adds their information from this web-to-lead page, it will link their lead record with the campaign based on the campaign ID field, with a responded status for that campaign.

NOTE: You are only able to associate the Lead with one Campaign through this method.

-Jager