In a real business word forcing customers to be signed up is one thing but to make it friendly you can have the box checked automatically when they do sign up for an account.
As dr byte said above
Code:
Admin->Configuration->Customer Details->Show Newsletter Checkbox
Show Newsletter Checkbox
0= off
1= Display Unchecked (DEFAULT)
2= Display Checked
choose option 2 and they will be automatically signed up when they click submit however Defaulting this to accepted may be in violation of certain regulations for your state or country
As far as I know this is the only way to do it.