Have a look at [the "How should I credit you?" entry ][1] in our legal FAQ. The gist from that entry is:
You need to credit OpenStreetMap and make it clear that the maps are available under our licence terms.
- If you are using OpenStreetMap **tiles** (e.g. Mapnik or Osmarender/tiles@home), we request that your credit reads at least "(c) OpenStreetMap and contributors, CC-BY-SA".
- If you are using OpenStreetMap **data only**, we request that your credit reads at least "Map data (c) OpenStreetMap and contributors, CC-BY-SA".
Where possible, '*OpenStreetMap*' should be hyperlinked to http://www.openstreetmap.org/, and CC-BY-SA to http://creativecommons.org/licenses/by-sa/2.0/ . If you are using a medium where links are not possible (e.g. a printed work), we suggest you direct your readers to www.openstreetmap.org (perhaps by expanding 'OpenStreetMap' to this full address) and to www.creativecommons.org.
Also note that this the license attribution might requirement might change, change once the data has been relicensed...
[1]: http://wiki.openstreetmap.org/wiki/Legal_FAQ#I_would_like_to_use_OpenStreetMap_maps._How_should_I_credit_you.3F