Unfortunately unlike the iPhone, there is no standard way to take a screenshot in Android Froyo 2.2 or Gingerbread 2.3 without rooting your phone. It's puzzling why Google did not build this feature into Android.
There is an application called "No Root Screenshot It" in the Android market that will let you take snapshots of your screen without rooting, but it requires you to download and install a free desktop application on your Windows or Mac. Once installed, you must run the desktop application with your phone attached to your computer. This will enable screenshots on your phone.
Once you've done this, you can disconnect your phone and take screenshots whenever you want. However, every time you restart your phone, you'll have to connect it to your computer and run the desktop application again. If you restart your phone frequently, this app may not be for you, as it may prove to be too inconvenient.
This is not ideal, but is the only way that I know of to take screenshots without rooting your phone. I don't recommend rooting your phone because there is risk of bricking your phone and it will void your warranty.
Do you think Ice Cream Sandwich will be made available to the MyTouch 4G? I really hate Gingerbread 2.3.4 on the MT4G because of the bugs. These are the big ones that drive me nuts:
1. HTC Sense crashes frequently. (no fix)
2. Apps moved to SD card lose their icons. (no fix, other than moving the app to phone memory)
3. Search feature in Native Contacts App does not work.