Date: 11/16/2008 12:21 am EDT
All three may work.
If you have the type of threshold that will allow enough height to place a piece of quarter round thats a great option.
Using a color matched caulk like Journeyman described is the most common method I use.
There are different types of transition profiles available but they can look clumsy,are usually special order and can pricey.
Hope this helps.