You can use Bing Maps for free (regardless of whether you use OpenStreetMap data or your own Bing Maps data) if you match http://www.microsoft.com/maps/product/terms.html
Assuming this is for a commercial application, it basically means that you are limited to 125,000 sessions or 500,000 transactions per year. More than that, and you will have to have a license. These restrictions do not apply if you are a trained / non-commercial user, although there are other restrictions on the volume of geocoding requests, etc.
Please note that just because you use OpenStreetMap data does not necessarily mean that you do not need a license - downloading the map control itself is considered a "transaction", so if you do this more than 500,000 times a year, you still need to get a license if you use it for commercial purposes.
If you are not sure, try sending an email maplic@microsoft.com
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