According to The Scotsman David likes to drink his whisky "long". Well that's what he ordered at the 2005 BAFTA's at any rate. In general bartending terms long or tall drinks are mixed drinks.
In this case long means getting the whisky with ginger beer and a squeeze of lime. When made with Famous Grouse the sponsor of the bar that night coincidentally, the drink is called a Ginger Grouse and you can get it already mixed and on tap in the UK. In the US in the 1870's this drink was called a Scottish Lass, no reflection on our masculine Time Lord, and then in the early 1900's it became known as a Mamie Taylor.
Today in the US it is much more common to mix whisky with ginger ale as you only find ginger beer in specialty stores here these days.
As most purist's would say whisky should always be "neat" I leave you with a great quote from Darrell Goodliffe- "Matt Smith is a pale imitation of David Tennant – it’s a bit like drinking whiskey and then a whiskey chaser."