That's because in the last 24 hours Twitter quietly increased the limit from 100 to 150 requests per hour (as seen in the update Twitter made to their developer wiki shown below). For a site that has had significant uptime issues in the past, hopefully this is a sign that they can control the firehose.
For a Twitter developer it might mean a little tweak to your code. It is important to keep track of the requests that get sent out, or you'll risk being blacklisted if you attempt to request beyond the limit. You can also get the requests remaining from the Twitter API without being "charged" for a request. High volume applications can request "whitelisting" which, if approved, increases the limit to 20,000 requests per hour.
The update to the rate limit does not effect the search API, which was already "quite a bit higher" than 150 per hour. Twitter does not publish the search limit, but recommends watching for a 503 error, a sign you need to throttle back.