If you want to use the tiles, rather than the data, there is a usage policy: [http://wiki.openstreetmap.org/wiki/Tile_usage_policy][1]
You are free to use the data and render your own tiles which can then be used freely, subject to the licence: [http://www.openstreetmap.org/copyright][2] Commercial use, withing within these terms, is perfectly acceptable.
[1]: http://wiki.openstreetmap.org/wiki/Tile_usage_policy
[2]: http://www.openstreetmap.org/copyright
If you want to use the tiles, rather than the data, there is a usage policy: [http://wiki.openstreetmap.org/wiki/Tile_usage_policy][1]
You are free to use the data and render your own tiles which can then be used freely, subject to the licence: [http://www.openstreetmap.org/copyright][2] Commercial use, withing these terms, is perfectly acceptable.
[1]: http://wiki.openstreetmap.org/wiki/Tile_usage_policy
[2]: http://www.openstreetmap.org/copyright