As of now- yes, there is no way to change it without external programs such as INVEdit/MCEdit or mods such as Single Player Commands.
However- if you can access the snapshots, 12w16a introduced commands from Multiplayer servers into Single Player. This includes the command '/gamemode' which, as obvious, can change you from Survival to Creative and vice-versa.
This is why i dont like the SP part of the game. you cant really do much with the game unless you have mods. What i usually do is run a multiplayer server (with closed 25565 port so noone can join) and play that way. that way atleast mods are easier to install and the command line work without a mod. it works pretty well.
Or if u wish and if your computer can handle it, you can open the 25565 port and keep your server secret. so if your ever anywhere else without your own computer and want to work on w.e it is your working on you can access it form anywhere.
Actually....come to think of it.. if you try this out, copy and paste the world you have in SP and paste it into the MP server folder. and set the config file's world name to your worlds name so it wont just make a brand new world.
/gamemode command will work then i believe. and you can switch to creative. Download the Bukkit server cuase mods are awesome.