usually no. Under Windows you'll have to remove java.exe and javac.exe (and probably javaws.exe) from your windows/system32 directory though, or you won't be able to set which JDK to use from the system path. On other OSs I've never encountered something like that in the installation.
You can have multiple versions of Java on your system with no problems at all. In fact, I have three versions of Java on my computer: 1.4.2, 5.0 and 6.0.
Ofcourse you have to take care that when you run a program that requires a specific Java version that you use the correct version, but that's not too difficult.
Ofcourse you have to take care that when you run a program that requires a specific Java version that you use the correct version, but that's not too difficult.