I'm really pleased to hear that finally Multimap has opened up it's API for use by developers "to innovate, experiment and try things out". Multimap has some fantastic data and probably the best maps in the UK who else can provide vector maps and a good range of raster maps? It's really great to see that this data can now be used to power mashups and other mapping experiements. I think Multimap through doing this will no doubt see some interesting applications created which will subsequently provide more business users to look at using their data over other suppliers so it should be win win.
The only limitation to this whole project is that the traffic is limited to 5% of the traffic generated on the pulbic site. If one user uses more than 1% of that, then Multimap will "be in touch to discuss how we can work together to help you grow". Which sounds like you'll be asked to open a commercial account. In any case, if you're looking to create something based around draggable maps then multimap's openAPI is well worth a look. In addition as Multimap support Mapstraction.com (mapstraction currently supports Multimaps paid service) you should be able to use that so that switching between providers becomes even easier.
For more info on Multimap's openAPI see http://www.multimap.com/openapi/