It's important to remember that Smith Rock is in the high desert of central Oregon. Most of the main climbing areas from Morning Glory wall all the way over to Christian Brothers are all south facing and therefore if the sun is out and it's at least 40 degrees, the climbing is extremely comfortable. Therefore, climbing can be great as early as February. Conversely, in the summer, if you climb in the shaded areas, it can be cool enough for fleece while the south facing walls are frying.
The most popular climbing months are March through November. In the hot summer months, don't forget the Lower Gorge, an area chock full of beautiful, splitter, basalt cracks.
If you are unsure as to what area is best to climb in at any time of year, go to Redpoint and ask... WE LOVE SPEWING BETA!