Just made the replacement of the battery for this iPhone 3G in 35 minutes. Very carefully of course.
For me the trickiest parts were with connector of the screen number 3, basically placing it back but it is easy though. Don't use your plastic tools to remove the battery but also don't use a very strong metal tool! it is sticked in, so patience and side by side a little push every time.
Love the feeling of going against this "programmed due dating" on devices.... against the corporate greed, no matter how much I love Apple products!!!
Enjoy and thanks iFixit KEEP IT UP!!!