As of today, "click-to-call" works this way: you provide your phone number, Google calls your number and connects you with the company. In the near future, Google and eBay will use their VoIP clients (Google Talk and Skype) for click-to-call.This move is interesting if you take into account that Google started to offer this year Google Checkout, a service that could evolve into a PayPal rival. Still, eBay is one of the largest advertisers on AdWords and the partnership will be good for both parties: Google will have more pages where to show ads, while eBay will makes sure Google will not expand too much into its area. It's also a good way to promote their VoIP clients.
Another interesting aspect of the agreement is that "the companies will also explore interoperability between Skype and Google Talk via open standards to enable text chat and online presence."