This has been said before, but remember guys, that the legality issue of applying pesticides varies from state to state, just like installing backflow devices. Therefore, if someone doing lawn maintenance in NY wants to know if he can spray some Roundup on some weeds at one of his accounts, he probably shouldn't be asking someone from Alaska, or Oregon. In as much as you may hate to do it, but you should look up your own state laws on the matter. They're bound to be on the Internet.
For example, here in Oregon, the law states under ORS (Oregon Revised Statutes) 634.106 (5):
LICENSING AND CERTIFICATION
634.106 Applicability of ORS 634.112 to 634.126 and 634.146. ORS 634.112 to 634.126 and 634.146 shall not apply to:
(5) Persons who do not advertise or publicly hold themselves out as being in the business of applying pesticides but whose main or principal work or business is the maintenance of small or home lawns, shrubs or gardens.
But again, look it up for your own state, and while you're at it, you better check your limitations even if you don't currently need a license for what your doing. More than likely, once you advertise or perform a "spraying service" you've crossed the line.