The Geo Unconference for Europe
When and Where: 12th-13th March 2010, The Guardian Offices, London, UK
This is not strictly an open source conference, but is pretty high profile for the Europe and particularly the UK and has been generating a lot of interest and "buzz". See their blog for details: http://wherecamp.eu/blog/. OSGeo participation/sponsorship would be a real chance to raise the profile of the organisation with the UK/European Geo-community. Sponsorship at any level (there are 3) entitles you to a presence on the conference web site and t-shirt, as well as a free pass to the event. Currently Yahoo Developer Network, SVG Netherlands, Axon-Active and Nestoria are sponsors.
I (JoCook) am already attending, though not presenting. I would be happy to continue my role as a "roving OSGeo ambassador" at the event, and could take along some (slightly limited due to travel) marketing material to give out.