Rear derailer!!!
 

I'm getting a road bike with Ultegra. The cassette is 11-21t.
If I add a new cassette with 12-25t or 12-27t, will I have to get another rear derailer with a longer 'arm' or will the short derailer that comes stock be able to handle the wrap? The crank will be a double: 53-39t.
Please forgive the lack of technical nomenclature.

Generally, a rear derailleur has two capacity specifications: 1) The largest inside cog it can accommodate; 2) The maximum number of teeth it can wrap. You are starting with standard-issue Tour de France gearing, absurdly high, unless your name is something like Armstrong, Pantani, etc. If you avoid cross-chaining, I'll bet you can use a 13-25 with your current derailleur. If chain wrap, but not jockey wheel clearance, is a problem, consider using a 50-39 in front with a 12-25 (I would very happily use 13-25) in back.

No, I'm no Armstrong. Capacity is the issue. To the best of my knowledge there are 2 Ultegra rear derailer sizes: Long and short. How do I know what the maximum capacity for a short? Can I use 12-25 with 53-39?

Per the Shimano website,

http://bike.shimano.com/road/ultegra/index.asp

either model can accommodate a 27T low gear. (Note that they have the wrap capacity specs reversed between the two models.) 53-39 / 12-25 will take the short-cage version up to its wrap limit; if you really want that 27T low gear, consider a 13 or 14T high or a smaller outer chainring.

Thanks for the help.