Setting up an off topic forum is a good idea as a community grows. Otherwise, random topics will pollute the "real" forums.

You can always block spidering or make the pages NOINDEX if you are concerned about theme dilution.

It's not an off-topic forum. I already have one.

What they want is a sub-forum on a specific topic that's not related to my niche.

A lot of the people in my niche are interested in that other niche. But it's completely unrelated.

Even if you don't see how the two niches are related, if there's a large crossover audience, you might as well take advantage of it to make your site more sticky. The fact that they've asked you to add the subforum on your site is a sign that you have a great community going that they don't want to leave. But it's going to be harder to keep them if they have to go off somewhere else to discuss a topic a lot of them are interested in (same problem with starting a second site for the other niche). If there really is a big crossover, you might find that some forums already carry both topics - something you might want to check.

I guess I'm having a hard time seeing a real down side. If you're concerned about keyword dilution, use mack's no-follow suggestion. If you think it might dilute your image as an authority on the main topic, or take too much of your time, give the moderator reins to one of those requesting the subforum and make it known that it belongs to them. Of course, you could set it up completely separate from your main forum, as a smaller forum or a blog, but don't make the two more than one click apart or you will lose people.

Be flexible. The two niches might drift apart eventually and the need for the subforum might go away. But if valued members are asking for it (and if it's not something I'd be uncomfortable having discussed on my website), I'd set it up in a heartbeat. Unless there's a problem you know of regarding the two specific niches, or something else I'm not thinking of.

If you think it might dilute your image as an authority on the main topic

Something like that.

Imagine if that was a board for business executives to deal with, say, managing their immediate subordinates. And they wanted a sub-forum about golf, because a lot of them like golfing.

I don't want to reject their request, but I don't want to dilute the razor-sharp targeting I have.

If your members want it, you should give them what they want.

They won't care at all about SEO issues. Block spidering or make those URLs NOINDEX.

I'd also suggest considering whether those pages might actually bring more traffic in if they were indexed. If this linkage between topics isn't unique to just your members, you might find that you start getting traffic on keywords that bring in new members with an interest in your primary topic. I've worked on forums that have ranked well for many primary terms but also generated a lot of traffic from rather odd long tail keywords, too.

I don't want to reject their request, but I don't want to dilute the razor-sharp targeting I have.

Using your example, would you see it as a problem if the forum started attracting golfers who weren't managers?

Just a couple of off-the-top-of-my-head thoughts -

Give the golfing subforum its own branding: "Mike's Golfing Corner" - where Mike is a requester who's agreed to run the subforum. Make Mike the authority, not you.

Group the golfing subforum with your off-topic forum, or even make it a subforum within the off-topic forum. Make it easier for members to discuss their requested topic apart from all the other "foo" but still keep it separate from the main discussion area.

All good suggestions, Beagle. I suppose part of the question is how attractive (commercially) the subtopic is. Maybe "golf" can be better monetized than "business". :)

That other topic can be monetized just as well (and probably better) as the main niche of my board.

I would intersect the niches and make it something like "golf talk by business execs for execs" to make it more unique than all the other golf sites out there.

But in this case, there is absolutely nothing that other niche has that would be specific to my main niche. In other words, my target audience does that other thing exactly the same way as all other people who do it too.

I know it will bring me more visitors that are searching for the other topic, but I'm not sure it's a good thing. Or rather, not sure if there are no downsides.