There are ways around it, specifically a /canceltarget macro:
/canceltarget..but it's incredibly annoying to have to use clumsy workarounds when the issue shouldn't be hard to fix (GC even said that himself).
/cast [target=mouseover]Wild Growth
Edit: the above macro may not be correct - if it doesn't work, try this one:
/cast [target=mouseover,noharm,exists] Wild Growth
This one comes from Restokin from a while ago. (PS - ignore my comments on that Restokin post - I thought my own macro at the time worked, but it turned out I was just lucky enough to go a long time without noticing the problem.)
After months and months, druids are still wondering what's going on. Why isn't it fixed? What's the hold-up?
Then GC said this today, and I had to laugh:
I was going to post that we fixed this, because we did. But I'm glad that I didn't because sometime during the 3.2 patch it broke again.
Sometimes when we don't address simple things head on, it's for reasons like this. We'll try to get it fixed (again) as soon as we understand what happened.
It's a serious issue that we want fixed.. but this really made me giggle. Glad they finally gave us word on what's going on, though.
Back to waiting!