|but most scopes have their signal ground connected to the earth ground. i'm
|not exactly sure why, but this is a fact. that the case is on earth ground
|is obvious, but why exactly the signal ground often is connected to this --
|i'd like somebody to please jump in here and tell me why. after all, this
|seems to be a major cause of trouble, so it seems to me there must be a
|really important reason for it -- and i should know it :)

[light bulb goes on]

I wonder if the connection of the scope ground to earth is to
avoid hazards which could otherwise result from connections
*BETWEEN* the ground leads on multiple probes?

Example: Bob is using one of the probes on his 'scope to look at
his television set.  Since the TV has a hot chassis, Bob puts a
"cheater" on the scope and is careful not to touch anything he
shouldn't.  After working on the thing for awhile, Bob decides to
go to lunch.

While Bob is at lunch, Joe needs to check out what signals are
present on a serial data cable.  Joe sees that probe #1 is some-
where in the TV set and doesn't want to interrupt what Bob was
doing with it, so he simply uses probe #2.  Unfortunately, when
he connects the probe #2 ground lead to the data cable, that now
makes the entire data cable and everything connected to it "hot"
(if it was floating) or blows stuff up (if it was ground-based).

Obviously, the operative words here are "don't do that", but this
type of situation may be what scope designers were trying to avoid
when they earthed the scope ground.