Just got Toshiba Satellite M645-S4045: Core I3, 4gigRam, 500gigHD. Room temp: 73 F. Just installed speed fan on it cause it seems like the fan runs when it's on but not doing anything. Under no stress all the readings are 45 - 50 C. Is that hot or what? I mean I'm used to my desktop quad running hot under stress, but for a laptop to run that hot under no stress seems odd. When I installed Windows updates and the load went up to about 25% on the processor, I got readings 55-60 C.
Take a look in the BIOS and ensure that the fan is indeed running. If you're lucky, it may just be a quiet fan. At idle, 45-50C is a bit hot, as in comparison, my M305 with a C2D runs around 35C at idle and when really stressed will crank up to the upper 50's. You can try using it on a flat hard surface instead of your lap (or however you use it) to see if the temps go down. If they do, a USB cooling pad may help you out.
Another thing to keep in mind is that laptop components, i.e. CPU's and such are made to take higher temps than desktop components. What sort of temps are you getting when the processor usage is 50% +?
If you think it could be a defective fan though, I'd just bring it back and save yourself a headache down the line.
Sounds a little on the hot side, tho many laptops do run a little warm. My MBP can get up to ~70c under load, but the fan will spool up too. Under normal load i can't hear it tho. Not unless its dead quiet and i put my head close. Even then i can't tell if im hearing the fan or hdd.
Is it a new laptop?
If not, most laptops are notorious for sucking cold air from underneath. That way it acts like a vacuum cleaner scooping up any dust and dirt on your desk, lap etc. After awhile dust builds up and reduces cooling efficiency. A compressor or can of compressed air can help blow some of the crud out.
Also remember your desktop is (normally) designed to run at full load all day. Laptops aren't.