I'm surprised nobody has mentioned this yet:
'Apt-get upgrade' is not he only way to upgrade packages with apt-get. To upgrade individual packages you simply install specifying unstable: apt=get -t unstable install <package>. Apt with then install that package from unstable ("Sid") or, if it is a package that is already installed, will upgrade the package and all required dependencies to the newest version.
You can use this method to selectively upgrade whatever you want without messing your system up with a total 'upgrade'. And Cuddles is right, an 'upgrade' (especially a 'distribution upgrade') can REALLY make a mess of things.
Don't wait for 'Sid' to make it to stable...the name 'Sid' is always used for 'unstable' and will never become a Debian 'version'.
And, yes, Synaptic, Kpackage, apt-get and dselect all end up using dpkg to do the actual install. Synaptic & Kpackage are gui based package systems which have the big advantage of giving you a 'shopping list' to chose from, dselect is a text-based Debian standard package selection system (does the same thing but much less user friendly), apt-get & dpkg are package installer/maintainers which will do what you tell them to. Apt- is slightly more user friendly, gives more notice of what it's going to do & what it's doing.