I'm one of the organizers of the Internet Identity Workshop and part of my job is asking for sponsorships to help support the workshop. One of the things that's a little surprising to me is how most companies expect that a sponsorship will get them a speaking slot.
IIW is organized around the idea of "open space" meaning that we set the agenda based on who comes and what they want to talk about as the first order of business. So, if you come, on Tuesday morning you'll see everyone gathered around in a circle writing session proposals on 5x8 inch cards that can be posted on the wall. So, my answer is always "Yes, you can have a speaking slot as long as you're there at 8:30 on Tuesday morning!"
Sponsorships help offset conference costs (and add to the bottom line of conference organizers) so they are a fact of life. We're used to advertisements supporting all kinds of things, but we usually like ads to be clearly identifiable as such. At conferences you attend, that just isn't so.
Often this works out OK. The speakers from the sponsor are relevant and interesting, but sometimes it's so blatantly obvious that you're being pitched that you just want to get up and leave. Personally, whenever a conference sponsor has someone speaking, I assume it was a paid slot. That's too bad, because it devalues the message.