Tax credits for PHEVs: Waste of time?

What about today's hybrid vehicles?

The US House of Representatives has passed the Renewable Energy and Energy Conservation Tax Act of 2008, H.R. 5351 that, if enacted, would provide tax credits for plug-in hybrid vehicles. The base credit would be worth $4000 for PHEVs with at least a 5 kWh battery pack. An additional $2000 in credits could be claimed by those with larger battery packs. Each kWh beyond 5 is worth an extra $200, up to an extra $2000.

The only problem, where are the plug-in hybrid vehicles?

Such a credit might push a few more hybrid owners towards converting their hybrids into plug-in hybrids, but wouldn't the best path towards PHEVs and significantly more fuel efficient vehicles be getting as many hybrid cars on the road today? Wouldn't tax credits for the Toyota Prius, for example, make more automakers address Prius success today, not 2 years from today?