One problem related to coolant leaking has been reported for the 2006 Toyota Prius. The most recently reported issues are listed below.
The check engine light comes on whenever outside temperature is above 45f or so. Took it to Toyota dealership and they said it was a faulty coolant flow control valve that was resulting in code p1121. The dealer said this was a safety issue since coolant can leak out, and recommended it be fixed ASAP at a cost of nearly $1000. This is not an uncommon problem for Prius's and an online search showed that Toyota did cover this under warranty (extension) for some model years up to 35k miles. I wrote to Toyota motor sales to ask if they would cover or assist with the repair cost, and if there was a hidden recall or warranty extension on this issue, and they responded that I should just try to negotiate a better price with the dealer. I think Toyota should cover this, it is a safety issue and should have been a recall.