Mostly it will not be sent.
I think it depends on why it's coming back with a "pending" message in the first place.
Usually, when I send a text that doesn't go through - it tries to send then tells me it has "failed".
If it's pending, it could be that the signal has reached somewhere along the network, but not been received by the device yet.I mean, I'm not 100% on this - but that's what it sounds like to me... If you need to delete and still have the option to do so, then maybe that means you've got it in time.