Why cannot update 5.1.1

I am having the same problem .... my itunes vrsion is 10.5.0 ... i'm running an iphone 3gs having firmware 4.1 .... i have the update in my itunes for firmware 5.1.1 .... but whenever i tend to upgrade and restore on itunes... i always get the error at the end of downloadation of firmware 5.1 ... 😟 .... tried many times but i cannot upgrade my phone ... please anyone help... 😟 😟

Is it because the next update (IOS 6) has already been sent out to all iphones and the update is no longer available for latecomers?

I have updated straight to IOS 6.0 through iTunes and it did it straight away and bypassed 5.1.1 so try it that way. Thanks to suggestions given through this site I now have an up to date iPhone. :-)