I generally think that paper and pencil are far superior to electronic machines for voting. Algorithms and computing can enable methods to support paper voting.

Clarification: serial number is mailed with the ballot, contains a signature (like two part keys for API f.ex.). You submit the serial for signing through authentication mechanism (verifying the voter). The signature can be either PKI or hash. This way you can validate serials, signatures and have them independent from the ballot after separation. If you have designated drop-off locations you insure the ballots are tamper-proof after being filled out (barring massive system-wide fraud).