the new lens is not the issue, it would not have caused this.
There seems to be a common problem with the 1000d, just read numerous reports with people having the same errors (its prob only 1 in a 1000 ... but its obviously not a one off), those that would not get swapped in the shop got sent back to canon and repaired under warrenty, all have had new mainboards fitted, so its obviously a common fault, might be worth playing your face a bit in the shop and seeing if they will go part way to changing to the 450 .. or a swap for a 400 and a bit of a refund if they have one in stock
, dont worry about the megapixels, my 350d is only 8mp but I can print A3 prints from it without any problem
hope thats some help
Phil