The 25th Amendment to the Constitution, passed in 1967 in the aftermath of Kennedy's assassination, specifies this order of succession:

* The Vice President Richard Cheney
* Speaker of the House John Dennis Hastert
* President pro tempore of the Senate1 Ted Stevens
* Secretary of State Condoleezza Rice
* Secretary of the Treasury John Snow
* Secretary of Defense Donald H. Rumsfeld
* Attorney General Alberto Gonzales
* Secretary of the Interior Gale A. Norton
* Secretary of Agriculture Mike Johanns
* Secretary of Commerce Carlos Gutierrez2
* Secretary of Labor Elaine Chao3
* Secretary of Health and Human Services Mike Leavitt
* Secretary of Housing and Urban Development Alphonso Jackson
* Secretary of Transportation Norman Yoshio Mineta
* Secretary of Energy Samuel Bodman
* Secretary of Education Margaret Spellings
* Secretary of Veterans Affairs Jim Nicholson
* Secretary of Homeland Security4 Michael Chertoff

Infoplease

Which is to say, it's Republicans all the way down (there are a couple of Democrats on the list, but they've all been appointed by Republican Bush). Succession takes a side trip into COngress as far as the Speaker and the Senate President Pro Tem, but after that comes back to the Executive branch.

I hadn't thought about it until DFM pointed it out, but I agree that succession is probably effectively stopped with the Speaker, who is automatically replaced. It could only proceed beyond that office if a significant part of Congress got wiped out simultaneously.

Personally I think we'd be better off with a parliamentary system, because it'd break the monopoly (or duopoly) on power of the two entrenched Parties. They've held power for so long that they've come to think that they are the government, or at least the only constituency the government should serve. Parliamentary systems are more fluid and provide a reasonable chance for third parties to emerge, giving the electorate meaningful choices. Very unlike how it works in America.

On another related note, if enough of their constituents wrote/phone expressing a certain opinion, would the representatives in Washington vote according to the constituents wishes?

That's how representative democracy is supposed to work, in theory. In reality it works to a greater or lesser degree depending on the other forces acting on politicians beyond their constituencies. Party discipline is the biggest enemy of representative democracy because it presents an honest politician with a conflict of interest: Obey the Party's wishes and get support in the next election; or defy the Party and face a Party-sponsored challenger. This phenomenon tends to exclude political moderates over time, reps who tend to listen more to their constituents than to press for an ideology, in favor of Party loyalists and dogmatists. Moderate Americans have effectively been disenfranchised by the political Parties...which makes it no surprise that American politics is characterized as massively polarized. It's not so much that Americans as a whole are polarized, as it is that only the polarized Americans have any voice and get any attention, because their outrage serves the interests of the Parties.

Is there a "recall" mechanism in the US? (in BC, if 40% of the registered voters in an electoral area sign a recall petition, a by-election can be called and if the incumbent loses, they are out).

Nope, unfortunately. Or maybe not--I think the Founders intentionally made it difficult to dislodge a government between elections to dampen extreme mood swings in the populace. It's a mixed blessing, because sometimes the people's instincts are correct and we need to act quickly; but other times we just get ourselves all worked up over nothing and the system's friction spares us regrets later. We've been spared the chaos some countries experience of having several governments in a year, at the price of reduced flexibility to respond to real problems.