This one's easier than you think it is, but you have to work backwards to get what you want. What I mean is, you do the text you want on the arch first. Assuming you have AI 10, you'll want to go under your Type tool and select the Path Type tool (it'll be the one with the T sloping downward at an angle). Click the arch and type. You might want to make sure your text is centered before you do this; go under the Paragraph tab of your Type palette (use Ctrl-T if you don't see it) and click the Center button (second from the left). Then type the first line of the text over the arch. Reset your Type tool to the default (the plain T) and you're good to go.