According to the StopTwitterSpam website, the folks at Twitter have updated their Help Page to reflect new changes that have gone into effect in regards to Following and Updating limits. It looks like these limitations have probably gone into effect to not only lessen the strain on the overall infrastructure of Twitter, but to also deal with spam. As far as the limitations go,

What are the limits, specifically?

The limits are based on multiple parameters–not a single metric. Because the limits are in part meant to curtail nefarious behavior, we are not revealing the specifics of how they are reached. Please note that these limits are sure to change as we figure out what works. We’ve taken a best guess to get started, but Twitter is still evolving and new uses are being invented all the time.

Good idea on their part as this will make it harder for Twitter spammers to reverse engineer the limitations. Hopefully, this does not adversely affect the majority of legitimate Twitter users and helps to make Twitter a more reliable service.