What I want to do is to generate a static image (e.g. a png) using python and using openstreetmap tiles as a background.
Mathplotlib and Basemap is almost what I\'m loo
You overlooked the "Export" tab at the OSM website, which is capable of generating a static image with the dimensions and map extents you want. Have a look at http://wiki.openstreetmap.org/wiki/Export
Please be advised that generating static images is a resource-intensive process, and the OSM sysadmins will frown upon you if you do a large number of requests or abuse this feature. Unfortunately this means you'll have to find another solution if you're trying to do lots of images.
By the way, the data you're plotting on top is properly projected into EPSG:3857 and not just raw lat/lon coordinates, right? Raw lat/lon data will look distorted at large zoom levels.