The 'Power Control' widget has five buttons for turning various functions on and off - including one controlling updating of applications (the circular one with two arrows on it). You can add this widget by long pressing (press and hold) somewhere on your home screen, then selecting 'Widget' from the menu that pops up, then 'Power Control'. I believe that if you press the 'refresh' button to turn it off, it should stop applications updating automatically, but they can still be refreshed manually if needed.
The widget also has handy buttons for easily turning on and off wifi, bluetooth and gps, and changing screen brightness. It takes up a whole row on the home screen though, so you might need to make some room.
edit - the below image shows the power control widget with updating turned on, and all other functions turned off, taken from: HTC Desire review at android.marvinlee.net: