I think politicians lie because they are motivated by
profit. Think about the way
the system is set up: Really, you only stand a chance of being elected to any
high-profile office if you can afford massive
campaigning. This means you are either extremely
independently wealthy (in which case you've already passed the test -- you must have been pretty profit-motivated to have
accumulated that much
wealth) or you're receiving massive campaign contributions from corporations who then own your ass.
The bottom line is, there is a very effective filter in place to prevent anyone whose integrity isn't for sale from running for office (or, if they do choose to run, the system just keeps them from participating fully. Look at what happened to Ralph Nader when he tried to join the debates).
I agree with almost everything mrichich said: Politicians could never tell the truth and still get elected. But I think the problem isn't only that people don't want to hear the truth, it's also that no one in office would tell you the truth even if you did want to hear it.
To everyone who can vote, I say: Who cares who you vote for for president? Bush and Gore (and Jean Chretien and Stockwell Day) are crooks. But vote in your local elections. This is where the possibility for real change exists and the candidates haven't yet been bought and paid for and you have a chance of finding someone with some integrity left. Change has to come from the bottom up.