Great post on scaling. (via Simon) Some good bits in the comments:

• We do TV advertising and have huge spikes within 3-5 seconds of the spot running. After maybe 20-30 seconds everything is back to normal again, with maybe +5-10% traffic (a normal variation).
• Times was an article that included links to the site. The article was one level off the home page (it was a section page). So, there you see the level increase and stay increased for about 20 hours or so (with some die down due to Times traffic behaviour). The Digg stuff, has a much shorter time frame. It is the life of an entry on Digg on the front-page. Anywhere from a few minutes to a few hours, but rarely longer than that.