I've had this issue with Handcent on my HTC Hero for quite a while (the wheel just spins and it takes 15-20 minutes or more before it sends).
I've noticed that the issue definitely seems related to the WiFi, specifically when there is a WiFi connection made or a WiFi connection dropped. If I'm texting as I'm leaving my office (where I was connected to WiFi) or if I'm texting as I arrive home (where it finds and starts to connect to that WiFi), those situations account for about 90% or more of the problems I've had with this. It would make sense if Handcent was connecting through WiFi, then got hung up if WiFi dropped in the middle of sending. But it's been happening for 6-9 months, so doesn't that pre-date Handcent working through WiFi? Maybe the cause on the LTEVOs is different, but on my Hero there's a definite correlation. Maybe it's something with the antennas both trying to work too hard at the same time an bogging down the processor, I don't know.
Not asking for help with my Hero, btw. I should be dumping that in favor of the LTEVO or GSIII within the next month.