A recent news item is the “hacking” of the Yahoo mailbox used by Sarah Palin (the Republican VP candidate) . It seems most likely that it was a simple social-engineering attack on the password reset process of Yahoo (although we are unlikely to learn the details unless the case comes to trial). The email address in question had been used for some time to avoid government data-retention legislation but had only been “hacked” after she was listed as the VP candidate. The reason of course is that most people don’t care much about who is the governor of one of the least populous US states.
Remote attack on a mailbox (which is what we presume happened) is only one possible problem. Another of course is that of the integrity of the staff at the ISP. While I know nothing about what happens inside Yahoo, I have observed instances of unethical actions by employees at some ISPs where I have previously worked, I have no doubt that such people would have read the email of a VP candidate without any thought if they had sufficient access to do so. If an ISP stores unencrypted passwords then the way things usually work is that the helpdesk people are granted read access to the password data so that they can login to customer accounts to reproduce problems – this is a benefit for the customers in terms of convenience. But that also means that they can read the email of any customer at any time. I believe that my account on Gmail (the only webmail service I use) is relatively safe. I’m sure that there are a huge number of people who are more important than me who use Gmail. But if I was ever considered to have a reasonable chance of becoming Prime Minister then I would avoid using a Gmail account as a precaution.
There is a rumoured Chinese proverb and curse in three parts:
May you live in interesting times
May you come to the attention of those in authority
May you find what you are looking for 
In terms of your email, everyone who has root access to the machine which stores it (which includes employees of the companies that provide warranty service to all the hardware in the server room) and every help-desk person who can login to your account to diagnose problems is in a position of authority. Being merely one of thousands of customers (or millions of customers for a larger service) is a measure of safety.
As for the “interesting times” issue, the Republican party is trying to keep the issue focussed on the wars instead of on the economy. The problem with basing a campaign on wars is that many people will come to the conclusion that the election is not about people merely losing some money, but people dying. This could be sufficient to convince people that the right thing to do is not to abide by the usual standards for ethical behavior when dealing with private data, but to instead try and find something that can be used to affect the result of an election.
Mail that is not encrypted (most mail isn’t) and which is not transferred with TLS (so few mail servers support TLS that it hardly seems worth the effort of implementing it) can be intercepted at many locations (the sending system and routers are two options). But the receiving system is the easiest location. A big advantage for a hostile party in getting mail from the receiving system is that it can be polled quickly (an external attacker could use an open wireless access-point and move on long before anyone could catch them) and that if it is polled from inside the company that runs the mail server there is almost never any useful audit trail (if a sysadmin logs in to a server 10 times a day for real work reasons, and 11th login to copy some files will not be noticed).
One of the problems with leaks of secret data is that it is often impossible to know whether they have happened. While there is public evidence of one attack on Sarah Palin’s Yahoo account, there is no evidence that it was the first attack. If someone had obtained the password (through an insider in Yahoo, or through a compromised client machine) then they could have been copying all the mail for months without being noticed.
It seems to me that your choice of ISP needs to be partly determined by how many hostile parties will want to access your mail and what resources they may be prepared to devote to it. For a significant political candidate using a government email address seems like the best option, with the alternative being to use a server owned and run by the political party in question, if you can have the staff fired for leaking your mail then your email will be a lot safer!