I think the main issue I would have with an online voting mechanism would be how to protect the Secret Ballot Act. Jim the suggestions you have made and the comparison to online banking etc all work on the premise of identifying a person before allowing them to vote, how does a system then "forget" the person to allow the vote into an anonymous pool for results? I am sure the technology exists to show only what we want it to but a determined person could find the digital trail is they really wanted to.
Added to that, the potential for attacks, etc, the need for independent verification, reasonable internet connection, etc. If someone is away for whatever reason a postal vote is readily available and is not difficult to obtain.
I have worked as a Presiding Officer and count staff. The system while manual and cumbersome does protect people. All the ballot papers have a serial number on them which is duplicated on the counterfoil, these are checked by the count staff, each ballot paper is stamped or perforated on the day of the poll, the stamp used is known to very few people before the day. These are both measures to prevent stuffing the ballot box. Anything with the wrong stamp would be discarded as a spoiled vote. For a poll result to be affected a significant number of ballots would have to be interfered with and this would also be noticed, the papers when checked would show they were invalid rather quickly. All work in a count centre is done under the watchful eye of official count staff, Returning officer and his team, tallymen, reporters, candidates and other interested parties, the opportunity to interfere with the count is slim.
Call me cynical but I think if the government, or more likely the advertisers who would pay good money for access to the voting preferences of a particular group of people had enough money the integrity of the ballot would be very much in doubt. Far greater risks in my opinion than the paper and pen model we currently use. Each and every ballot is verifiable back to the polling station/presiding officer it was issued to and each table that is set up has a tally of how many votes it issued. And in the event of a close result there is the opportunity to recount and re-examine each vote, which is not available with e-voting.