I have adsense on my forum, which receives not quite 2500 hits a day, yet I don't get many clicks. I will receive maybe 4 clicks a day for 18 cents. I believe the main reason for this a) I don't get a lot of unique visitors (around 100) and b) most of those visitors are return visitors, which always have a lot CTR (they kind of become immune to the ads, so to speak).

Anyway, would I be better off switching to a different advertising network so I can get paid in CPM, at least until my forum grows larger? Even if I can only get about $.15 cpm it would be more than I'm getting now from adsense.

Yes, CPC ads sucks, and only CPM can pay well in forums. I recommend you can choose Adbrite which is a great CPM network.

I registered at adbrite, but I don't get any CPM offers. I only get CPC offers, and they are only around 5 cents a click. I'll stick with google over that.

I applied to a few other networks, but they have all rejected my site, mainly because of "unmoderated content"

Keep trying other networks. There are a ton of them. And yes you need CPM or maybe even some sort of product depending on your forum topic. Like if your forum is a gamer forum, sign up with CJ and put game ads up....or check out widgetbucks...
Good Luck