Funny you should ask.
I literally just contemplated this exact situation not three weeks ago. I also, have a 60CS, I wanted to upgrade my maps. So I did some research and found after talking with Garmin, they will not be making another update after the current version that is available, which is 2009.
The 60CS' replacement model (60CSx) is compatible with the new version of software (City Navigator NT) They have decided to go with this new software because they way the device reads and stores the information, it can store more info with less space used.
I however, really like my CS and it still works flawlessly, so i decided to keep it. I bought the 2009 update and will use it until I feel it is time to upgrade.
I was concerned that the 56mb of memory our unit has would be insufficient to fit enough maps but this was no issue at all. (for me of course)
I hope this helps and maybe answers some questions for you. When searching for the update, Garmin does have it for $100 but when I did a Google search, I found it for $70 with free shipping.