And as far North as Sequim, WA, aka "The Blue Hole" named by pilots flying over the area from Seattle to Vancouver BC who alwas noted how the sky opened up whence flying over Sequim. The average annual rainfall is around 16 inches and the average year around climate is in the high 60's and it seldom snows.
Unlike many opinions so far Banana Belts are not exclusive to the California Coast or the Pacific Coast for that matter. There's BB's in the Black Hills and even in areas of Montana and on the East Coast. Take a look at the Wikipedia definition provided by Deborah below.
Call me a monkey, but yes, I live here in Forestville, heart of the Sonoma County 'banana belt.' While it is not a tropical climate by any means, it is extremely comfortable, great for growing plants, and wonderful for outdoor activities. We do reach the high summer temps in the low 100 range for a few days each summer, but the beauty is that we cool off at night with the coastal breezes, usually after 7pm even on the hottest days. I have lived in this town for 14 years, west county for over 20 years, and the northern Sebastopol/Forestville area just can't be beat.
This would include the areas both east and west of the San Juan Islands. You'll hear quite frequently that Sequim is in the "banana belt." Sequim is on the Olympic Peninsula west of the San Juans. You'll also hear Anacortes being in the "banana belt" and it's located to the east of the San Juans.
I have heard of banana belts, and knew the term referred to a specific area that enjoyed a particular climate. I assumed it meant warmer, but have heard the term banana belt applied to several areas outside of CA. This thread inspired my curiosity, and I still am not totally clear. But....here is an interesting link with short reading