I made a really simple one with an OpenOffice spreadsheet. In the first sheet I separated all of my gear into different categories, weighed them in grams, and had the spreadsheet convert it to ounces. In the second sheet I had the same categories but left the spaces in between blank so I could use it to add up the gear for specific trips. I just copy the gear I'm taking from the 'Source' sheet into the 'Pack' sheet. I have totals at the bottom of the sheet for base weight, weight worn, and FSO, all in grams, oz, and lbs.
I've even started working on another sheet for nutrition, with weights (g and oz), calories, fat, cal/oz, etc.
It really isn't a big deal to do it. The little summation button at the top saves you a lot of M5+N5+...