Apple and AT&T have started sending out $40 checks to U.S. customers who bought or ordered a first-generation iPad with 3G capabilities before June 7, 2010. A ruling was made in September in a class action lawsuit stemming from AT&T’s elimination of an advertised unlimited data plan about a month after it was introduced. The checks have started to arrive in customers’ mailboxes, as per 9to5Mac. AT&T is also offering a $20/month…