At issue is the rule that, once a certain vehicle reaches 200,000 in cumulative units sold, it is no longer eligible for a tax credit. The all-electric Nissan Leaf is furthest along that path, though Nissan has already gone on record saying it's going to negotiate with the feds to get that number increased.
Right now, fewer than a third of the plug-in vehicles available in the US are listed on the IRS page of tax-credit-qualifying vehicles, with models such as the Honda Fit EV and Kia Soul EV excluded from that list. And as far as cumulative numbers, things don't match up there, either. The IRS' numbers are quite a ways from Ford's in terms of plug-in vehicles sold, for example. For more, start with our original post and then get the update from GCR.