MapBox provide hosted tiles, beautiful designed custom tiles and a bunch of nice tools (including iD, the in-browser editor). OpenStreetMap provide the community-maintained dataset. MapBox have been very supportive of the OSM community in terms of donations, sponsorship of events, use of office space and so on.
The point is if you are Foursquare and you want to start using OpenStreetMap, MapBox will give you commercial support and design services and so on. OpenStreetMap (the community, the non-profit Foundation etc.) won't.
As someone who sits in #osm and #osm-dev on irc.oftc.net and helps at least one person a day, I'd like to point out that the community will most definitely give you support. MapBox certainly makes it easier to pay someone, but the community isn't completely shut off from the world :).
Mapbox also pays Artem Pavlenko, the lead developer of Mapnik [1], which has for a while been the main tile renderer used by OSM. Their interests there align nicely, because Mapnik is also the rendering backend for Mapbox's TileMill software.
