So you stopped your banner ad campaigns a long time ago because the click through rate sucks and the conversion rate was even worse?
The problem is - you might be leaving money on the table.
In a couple sessions at SES, I've been hearing about how banner ads can drive people to search. So while they may not be clicking on your display ads and converting off of them, they may send people to a search engine to find your product or service.
What you'll want to do is set up the banner ad and then watch your analytics to see if you have increased searches and if those searches are related to the banner ad. You'll want to take all of that into consideration when creating your landing page, bidding on keywords in paid search, and writing the paid search ad.
The only caveat I would add is to place your display ads carefully. Social network users have said they don't find the ads targeted to them, so other content networks may prove more successful.
Microsoft Top Display Advertiser for June 2008
Online Publishers Turning to Ad Networks to Sell Unused Inventory
AdBrite Adds CPC Option for Banner Ads
Google AdWords Display Ad Builder Gets Four Updates
Q Interactive Launches Display Ad Network with Predictive Behavioral Targeting