I have a kegging set up for me beer, and I also have a carbonator cap for water, soda, & taking 2 liters of beer to go.
It works great! You will need to shake it, put in more co2, shake, repeat several more times or just leave it under pressure for a while. Carbonation doesn't happen immediately so you either have to shake it a lot to force carb it or let it sit under pressure for a few days.
If you can find a co2 tank & regulator on craigslist, you will save some cash, but it will cost about $130 (http://stores.kegconnection.com/Categories.bok?category=*Soda%2C+Soft+Drink%3ASoda +Kits%2C+Carbonating)
Don't get the tiny co2 tank since when you get the co2 filled, you aren't paying much for the co2, you are paying for them to fill it up. 5lbs of co2 will last you a long time if you are just using it for soda. To fill it up costs around $15 (depends on where you live. Costs $17 around Boston).