I just read something in the FAQ part 2 that might shed some light on casting into combat. The question was about the spell from the colloge of metal called rule of burning iron and it states that it can be cast into combat even though the spell itself does not specifically state it can be cast into combat. This makes it fuzzy on rather or not tree singing can be.

I just thought of a good reason to do flee and let them hit another unit. If I have one of the special characters with a unit of skinks right in front of him draw out someone and then let him stand there and take the charge. He gets to strike first when people assault him so it would work out. Plus he gets to make you reroll successfull to wound hits.

The casting into combat rules seem pretty crystal clear to me.

"Wizards cannot cast spells at units engaged in close combat unless the spell only affects the caster himself or the spell's description specifies otherwise." BRB pg 107.

Rule of Burning Iron is a spell that had been argued about for quite some time about the ability to cast it into combat (or even into games on the neighboring table). I'm actually rather relieved that they got around to answering this one finally. The reason that there is an exception on this spell is the spell specifically states "No targeting restrictions apply except that the wizard needs line of sight to the target." It seems clear that GW has ruled that a spell not being able to be cast into combat is a targeting restriction and would be included in the catch-all phrase of no targeting restrictions apply.

I don't believe that tree singing states anywhere that it ignores all targeting restrictions. Simply having no requirements for range or line of sight doesn't mean that it completely ignores any and all other restrictions altogether. Even a spell like, say, foot of Gork which states that it stomps "any one enemy unit on the table" may not be cast on units in close combat because it isn't specified in the spell description.