Tried that once, never again. Unless you pack a recharger it's not going to last 7 days. Cellphone batteries aren't robust, and you can't just pop in a new one when the old one dies. Plus, if you get into trouble you've killed your method of calling out. I work SAR, and have had to search for more than one person whose cellphone died before we could get a fix on them, usually because they were using the damn things to play games, or as a flashlight.

I don't know what GPS unit you have, but a basic low-cost (lower cost?) unit that just does GPS is more beneficial than the large units that have cameras and other fancy doodads (like the Garmin Montana). Weight is less, battery life better, and a spare battery is only a few ounces. Or, you can usually pop in AA's and just count the weight as backups for your flashlight, etc.