Just had a closer look at this one - Tilia americana was added in 2003 so it should have been there for a long time already. It's part of the "BRC list of additional vascular plant names" checklist. I am issuing an update to the NBN Gateway in the coming days but I don't think it will change the situation with regard to this taxon :/
Maybe one for Mike to check, to see where it has gone in the Recorder taxonomy?
Chris Raper, Manager of the UK Species Inventory, Angela Marmont Centre for UK Biodiversity,
Natural History Museum, Cromwell Road, London, SW7 5BD. (tel: 020 7942 5894)
also Tachinid Recording Scheme (http://tachinidae.org.uk/)