Steam submission

Unfortunately Steam's FAQ has changed since the release of Greenlight, so I can't really find out what the specific processes were before its release. However, it seems as though one of the first steps would be to email Steamworks about publishing at steamworks@valvesoftware.com.

Link to Steam FAQ.

Steamworks Brochure.

Content delivery software appeals to indie developers due to low to no costs for updating, activation, bandwidth, etc.

These costs and advantages apply to games on steam post Greenlight.

Microsoft's Indie Game section of XBL requires an annual membership fee to submit games, according to their FAQ.

http://xbox.create.msdn.com/en-us/home/faq