Nice post. You raise some good issues.
The debate between charging by the hour and charging by the job continues in our company. We're very firmly on the fence right now, but will probably end up staying with our current, hybrid solution.
We started out charging by the hour. As you've mentioned, this can appear to be inequitable to the client, so we switched to charging by the job.
Then came the time leeches. So we started folding in "by the hour" charges on each "by the job" contract for those clients who can't stop with the long phone calls -- or the "simple" email questions that take an involved answer.
Each job has such unique qualities that we often start out charging by the hour until both parties are clear about exactly what services are going to be involved. We then nail those precise services down with a "by the job" contract, always including a few clauses to handle the time leeches. And deadlines with penalties for each party (that can unclog the raw material gathering process very nicely.)
Another area we've struggled with is the apparent "prospect" who only wants to pick our brains and then take the job in-house wiuth their new found knowledge. We're setting up paid Evaluation Services to break the ice with new prospects but still make it clear that our time and knowledge is valuable.
Offering copy writing for the web is a good thing. There is one challenge here - good copy is still not valued as highly online as it is offline. Just tour a few sites and the proof is right in front of your eyes.
It is my opinion that copy is THE major untapped resource in e-commerce. The web allows a company to establish a much more human voice, but how many do? When someone puts down a brochure, they may pick it up again -- but that's not so likely when they leave weak copy on the web.
I've been writing copy since 1993, and offline I can command, well, let's say a very healthy rate. One of the elements surrounding online copy that keeps me from taking on more is the way it is undervalued. I will lower my rate for a client if I'm providing other web services, but I won't go dirt cheap. I've honed my skills, and add in a healthy dose of SEO to my writing. That's worth a lot. Good copy, well tuned to the audience, brings in healthy ROI. Thats's why offline copy can command good fees.
So I feel it's important to know the value of what you offer and fight the good fight until the clicks people wake up to what the bricks people already know.
(Sorry for that overworked metaphor, but it's late, you know?)