So today I decided to try out my Adafruit Ultimate GPS Logger shield. (http://www.adafruit.com/products/1272)
I carefully solder all 32 header pins (to match up with my Arduino UNO R3). I take my time, soldering under a magnifier to get all of them nice and cleanly soldered.
When it's all set, I plug it in and run the Arduino IDE.. fire up the serial monitor at 9600 baud.. and I see a line of what looks like GPS data, which is soon scrolled off the screen by a stream of lines containing only 3-digit numbers. This isn't what I should see per the instructions!
I scratch my head for a minute, and then remember the last sketch uploaded into that Arduino was reading an IR sensor that I had connected to one of the analog inputs and dumping the readings to the serial monitor. So I decide to write a 'blank' sketch and upload it.
Problem solved. GPS data starts filling the screen exactly like the example in the instructions.
Just thought I'd share this for anyone else who might fire up a GPS shield for the first time using an Arduino that's had code uploaded previously! Rookie mistake, but I'm glad that I figured it out in under 30 seconds. :)