I haven´t noticed a backfocusing problem when I shoot my lens on my 450d. If the lens is under a year old it should still be within warranty and I would call Canon so they could adjust your camera to your lens.

Do you use focus and recompose ? Can you post some examples?

I don't know if this fits in with your 'problem' x3adnila, but I have found that the super fantastic 55-250, does have a couple of shortfalls ( heaven forbid for writing sacrilegious statements about this lens!).
1. AI servo and the canon 55-250 are not best buddies.
2. When close focusing,in lowish light conditions, hunting will occur as, for some reason, low light through the 55-250,at close focus, puts the cameras AF into a bit of a panic.

ai servo works fine for me and i find this lens way better at low light focusing then the 50mm F1.8 and that says a lot maby its a issue with your camera or a setting or your sample.

I agree that Al servo and this lens don't play well together...I've got probably better than 40,000 shots with this lens and have to say it's just a wonderful lens and it's slow...When I'm having trouble with AF and this goes for any camera then choose your own Focus point. There is a chance you need calibration but if you think this do the tests to confirm it. There are a lot on the internet to choose from.
Sue

wel i always shoot single point maby thats why i dont have any issues :)

I have had sharpness issues and focussing issues when shooting with this lens. In most cases where I found that the AF looked doubtful I was shooting at 1/20 seconds hand held at 250mm. Once I started taking tcare of the 1/focal length* Crop factor shutter speed I started getting more satisfactory results.

However I'd say 80 percent of the time the focus is acceptably sharp.

I do not think that this is a stellar lens as opposed to my 100 mm macro which is outstanding. However for what this lens cost me it delivers very nicely.

Probably on a better body like the 40D (AF is better than my 450D) this lens would give more keepers.

Birds n bugs...do you think the focus is poor in these shot ? I ask because I find these acceptable but you may not think its good enough depending on your experience and better performance that you may have seen.

that focus is acceptble even tho i think the first shot is focussed on the gras in front of the bird rather then the bird itself the second one is hard to judge as i cant know wich focus point you used.

The second shot was cnetre point focus on the eye and cropped quite heavily.

Thie first shot was focussed and the bird would not stay still....so perhaps I shud attribute it to my lack of skill to cpture this sort of subject.