I've found the same issue - die @ about 30C reading -10. The temperature does go up when the chip is warmed so it may just be a offset error.
Please refer to the PSoC5 ES1 Silicon Errata document over here - http://www.cypress.com/?rID=43758
According to the Errata No - 2, Temperature sensor is not factory trimmed. That is why you are getting a very big variations in the output. This fix will be available in ES2.