Negotiated Link Speed only 1.5Gbps for 256GB SSD on 15" mid-2009 MBP?

I just installed a new Crucial C300 256GB SSD in my mid-'09 MacBook Pro (2.66GHz Core 2 Duo w/ 4GB RAM). I didn't have any issues with the install but I did notice that the negotiated link speed shown in System Profiler for the drive (under Hardware - Serial ATA) is only 1.5Gbps when 3Gbps is possible and the SSD supports 6Gbps transfers.

Here's what's really weird -- if I reset the computer's SMC, the correct negotiated link speed of 3Gbps shows up after booting...until I reboot the computer at which point it drops back down to 1.5Gbps. What the heck is going on here? How do I get the SMC to "stick" so the drive doesn't slow down after a reboot?

FWIW I'm running EFI version 1.7 so SATA-II speeds should be possible. I have read about the issues that some are having with mid-'09 MBPs and non-stock hard drives so I suppose this might be related...

Thanks in advance for any pointers!