There is not a lens available today that will give you an effective fisheye effect on the 1.6 XT. Go full frame.
Agreed on the first part; Canon shows little interest in providing more advanced or specialized wide angle lenses for its EF-S mount DSLRs.
For some alternatives to paying several thousand dollars more for a Canon 35mm film format DSLR, you could consider
- a Nikon DX format SLR with the Nikon DX 10.5mm 180º fisheye lens.
- an Olympus (or Panasonic) 4/3 format SLR with the 4/3 format 8mm 180º fish-eye that Olympus has coming soon.http://www.olympusamerica.com/e1/sys_lens_8mm.asp
- a Pentax or Samsung digital SLR with the DA 10-17mm 180º-100º fisheye zoom that Pentax has recently announced.http://www.pentaximaging.com/products/prod...ubsection--null
Above all, do not take the limitations that Canon currently imposes on its EF-S offerings as limitations of the mainstream digital SLR formats in general!