I've seen other people with this problem, and I was just wondering if there was a fix for it yet. There are areas of striped land with a random biome and this also creates invisible trees if they are those stripes. It looks something like this: http://i.imgur.com/xg1eE.jpg Thanks.
Yeah, I get this too in both Vanilla and Bukkit. I think it is there way to make the grass more varied...
I noticed that this sometimes happens when you generate new land, but it's mostly client-side (communication issue between server and client). If you reconnect it can fix most of that.