Starwood tend to run their Starpoints sale twice a year. You now can buy up to 20000 at 3.5 cents per point before discount. This is also the yearly maximum that individual can buy from Starwood per account per year.
Once the points have been deposited to your Starwood Preferred Guest account, you are free to use them the way you see fit.
20K Starpoints turn to 25K miles on most airline partner programs. This would be around 2.24 cents per airline mile. Not the best price around, but if you have maxed out other options, it is bearable.
Note that this offer is valid from July 20th to August 31st, 2012.