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My Subaru burns 0.05-0.08gal more fuel to get warmed up in the winter (per ScanGauge), regardless of the length of the trip. On a 0.3gal (each way) commute, that is a huge number of em pee gees.
The amount of fuel a block heater would save has a CO2 output similar to 0.3-0.5KWh of grid-average electricity. If the Subaru's block heater uses more than 500Watt-hours, it's greener (in terms of CO2) to not use it. I'm guessing the Prius uses less gas than a Subaru to get warmed up, so unless your block heater can do the job in less than an hour, I'm pretty sure it's going to produce more CO2 than just letting the gas engine warm itself up.
Bottom line: With block heaters, PHEVs, and deep-cycle alternator replacements, you can save a lot of gas, but the environmental benefit of plugging in is questionable on US grid average electricity. Of course, if you opt to purchase low-carbon energy from your utility company, or if you care more about energy security or HC emissions than CO2, then you'll want to plug in.
Another interesting calculation: one deep-cycle marine battery probably has enough energy to heat your engine block. You could recharge it with the alternator (or DC-DC converter on a Prius), or recharge it at work. However, the 65lbs of dead weight won't help the FE.
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