The minimum-wage ACORN worker (or whoever it was) who filled in fake voter registration applications SHOULD be prosecuted, mainly to discourage other shirkers and pranksters from pulling similar stunts (wasting ACORN's money, taxpayers' money, etc., and gumming up the important work of letting citizens vote). But that offense probably is, and should be, a misdemeanor, punished by a fine and/or community service.
The important crimes that are, or at least should be, felonies and that really need to be vigorously prosecuted are things like these:
~tampering with electronic voting (or vote-counting) machine software
~falsely "notifying" or advising citizens of changes in precincts, changes of election dates, etc.
~voter intimidation in the form of harassment of would-be voters by "challenging" their eligibility at polling places without probable cause.   (Note: someone's race, apparent ethnicity, gender, manner of dress, etc., or the location of the polling place is not probable cause for questioning someone's eligibility)
~voter intimidation by fraud, e.g., sending messages to students falsely informing them that if they vote at their campus location they will lose their scholarship eligibliity; sending messages to urban residents that if they have outstanding parking tickets or are late on child-support payments they will be arrested on the spot if they try to vote.
~deliberately omitting or distorting the name or party affiliation of a candidate on the ballot (e.g. leaving Bob Barr's name off the ballot, listing Barack Obama as "Barack Osama," or listing John McCain as an Independent).
~state election officials deliberately allocating disproportionately more voting machines to suburban precincts than to urban ones.

The various and sundry serious election fraud tactics are not going to stop or even slow down until some people face serious jail time for them.   (Fines won't do it, because they don't matter to rich people.)