Good stuff, Monte. I especially like the search results angle.
Scot, no reason not to try all/most of what Monte suggested. You'll learn stuff.
I would add that A/B testing messaging via landing page variants and search ads is a powerful combo. Tactically, I like Unbounce for A/B testing but there are a bunch of others out there. There's also a new product out called QuickMVP that makes it uber-simple to run small tests... they even take care of the AdWords processing.
I understand that your question is principally about validating marketing messages. However, since your title is about validating a market (and not just market messaging) I'll add here that customer development is about the most valuable things you can do early on in a startup. There is simply no substitute for actually talking to your potential customers... a lot of them... often... face to face. For you, that would mean talking to the potential users as well as the decision makers at small/medium businesses.
It is remarkable what you'll learn from having an old fashioned conversation with a customer that you would never have learned in any other way. It's not just about validating your assumptions or finding customers that like what you're doing. It's about finding out from them what they would do differently, to make it better... it's about what else they'd like to see that you didn't think of... it's about hearing the visceral pain in their voice. In the process, you'll be experimenting with your messaging with each new conversation, refining it each time until it resonates like the bells of St. Marks.
Approach customer interviews with humility and you will be rewarded. Don't tell them what your solution is and then try to talk them into liking it. Ask them to explain their problem, how they currently solve it, what their ideal solution would look like -- before you've told them what your solution is!
Assume you're on the wrong track from the start and seek their help to correct you. In fact, if they like your idea then try to talk them out of liking it. If you can't talk them out of liking it then you just might have something.
How many is a lot? Well, it's never enough. I've had more than 150 face-to-face customer conversation about my current startup; and I'm no where near done. I learn something new with virtually every new conversation. Just when I think I've heard all the new ideas, the twists on a theme, the novel approaches... here comes another. Seriously, it amazes me every time.
Find aggregation points to make your convos more efficient. Are there annual meetings, industry trade shows, meetups, seminars, etc. where the market sectors come together en masse? Yes, use LinkedIn too, but to find out who your connected with in your target market and then ask for an intro or just ask to have coffee directly. Since your not selling anything to them (you're just doing research) you'll find plenty who will let you buy them a cup of joe.
There is a ton of good stuff written out there about how to do customer development interviews. In the end, however, just focus on have a real conversation and a humble exploration of the problem set you're going after. If you do that you'll get amazing results.