Israel on Thursday released some 200 security prisoners due to overcrowding in prisons, Israel Radio reported.The overcrowding reason seems unlikely. After all, the number of security prisoners today is roughly half what it was in 2007. Ha'aretz says it was a gesture for Ramadan (and gives more background behind Hassan Yusuf.)
Among those freed was Hassan Yusuf, a senior member of Hamas in the Gaza Strip. Yusuf, who had served six years in prison for his activities as part of the terrorist organization, was scheduled to be released in a month and-a-half.
It does look like most of the released prisoners did serve all of, or close to, their full terms, so this does not seem to be related to any Shalit negotiations, unfortunately.
Assuming that Israel released prisoners in May, June and July at the rates they did earlier this year, this means that there are now less than 5000 security prisoners in Israeli prisons.