I've been using Zenphoto to present video clips - up to this point the clips have been in .mov format using the Sorenson 3 encoder.

Now I've switched to .mp4 files using H.264, but Zenphoto appears to have problems with this. On the individual video page, rather than bringing up the player, I see a message saying "No flash player installed." Is there a fix for this, or are H.264 mp4's not compatible with the current ZP video player?

Flowplayer says it does: http://flowplayer.org/documentation/technical-facts.html
That message actually does only appear if you have no flash player enabled. Make sure you have and also enabled the video class plugin.

"That message actually does only appear if you have no flash player enabled."

Do you mean no flash player enabled for my OS/browser, or in Zenphoto? I definitely do have flash enabled in my browser. If you mean in Zenphoto, where is the option to do this?

Ah, I see... you meant no flash plugin enabled in Zenphoto admin. Thanks. I've enabled flvplayer and they do play now. Though I have to say, it's a shame flowplayer and flvplayer both display their branding watermarks when you play videos...

Regarding the branding: To disable that you need to buy a commercial licence. See the sites of both players about that. It's the decision of their developers and I think it is a valid one. You can't get everything for free.... (flvplayer is only free for none commercial purposes even with branding, that is why we do not support that plugin anymore).