Anyone know what could keep causing this to come on all the time, the first time it came n my ECU was replaced and now a week later its on again, what could it be now?

As lame as this may sound, sometimes it comes on when you don't have the gas cap seated well. When you put it back on after filling up, make sure it clicks a few times. After a few times of starting the truck, oftentimes the light goes away.

Being as how you just had that other stuff replaced, though, what I said above might not help at all.

i checked the gas but its still on...

I have had the same thing happen a couple of times on my '01. I talked to my mechanic (family friend) and he says that the indicator light comes on for emission control reasons. Sometimes caused by a mis-fire (ie. mis-start) or as previously said a loose gas cap. It should go out after 6 or so cold starts. Thats what has happened with mine anyway. If your ride is running well I would give it time and it should go out. Anyone else got any ideas?

You need to have the codes read to find out what the problem is.
And you need to read up in the problems/annoyances section

mine used to do this in the summer all the time.
I always used to squeeze as much gas in as I could now I quit filling when the pump cuts off havent seen the dreaded light in some time...do you "top off" or fill til the pump cuts off?

Topping off can get fuel into the charcoal vapor canister and that will in turn allow the charcoal to get into other area of the vapor containment systems and trigger a light. Don't do it.

yeah what brent said