Like happygeek, I'm also a freelance writer. Spent 20 years in major advertising agencies—I was a creative director on accounts like AT&T, Microsoft, and IBM, before going freelance in '07. I agree with everything he says. I just want to add a few pointers, for companies who may or may not have the budget for a good freelancer.
First of all, look at your team. At least one person should have lead responsibility for marketing and sales. That person should, at the very least, be able to write clearly. Clarity is more important for early stage companies than cleverness. If that person can write your value prop so everyone understands it, you're ahead of most.
If you are going to hire someine, make sure you know what you want your writer to do. If it's a website, what kind of website is it. Sales? Branding? Service? If it's an ad campaign, who are you trying to reach, what do you want them to do and why should they do it? Hiring a website writer because you like a clever ad they may have done is a mistake. A seasoned writer can walk you through these strategic steps before putting pen to paper and, in fact, that may be worth more valuable to you in the long run than the writing.
Also, make sure you have a solid business case to justify the writer's fees. I know of an incubator-stage startup that wanted their blog to be the be-all-end-all of expertise …