One thing I love about blogs is the instant feedback aspects. While certainly comments on a blog are far from scientific you can get some great anecdotal responses that can help you see problems in a new way or provide some real-world data to backup one position or another. So my tip to PMs out there is to think carefully about what question you want to know the answer to and blog it! I have really benefited from the feedback I have gotten on my blog and I know several specific instances where the feedback changed my mind and\or helped me impact the product in tangible way.
Of course, you should still use the official channels such as SDRs, usability studies, market opportunity analysis, etc… but many of those are quite general and long lead. Involve the community in the heat of the moment… I would not use blog feedback as the only data point, but it is certainly a data point you should use. You do not have because you do not ask…
This topic pops to mind as I watch a couple of masters use this technique to great effect… how can any PM read these comments and not have it effect their view of the solution space!
Don Box on HTTP, XML, REST, and $100