OK first off the misconception on data is that there is "NO" 5 GB cap on data, if your using the phone as it's intended there is no cap on the amount of data that you use. The key word here folks is the "as it's intended" part. If you use your droid and stream music to it for a couple hours everyday while your at work and also watch some youtube movies in HD, and surf the web, read your email; the likely hood of you coming close to that 5GB or even over is pretty big. Verizon knows this and that's why your plan is for unlimited data.
Where you will come into problems is that once you go over that 5GB, you do kinda get eyed with what kind of data you use, and they may even want to look into it further, if you are using the device as intended, there isn't going to be a problem at all. If though for some reason they catch you downloading stuff, or right this second for some, watching flash movies, and just being completely obvious that this isn't the phone doing this, they have all the right in the world to charge you for the data used.