The rarer the event, the more likely that things can disappoint...
1) JPL continue to have trouble with the focus of their High Resolution camera onboard the Deep Impact craft. A Medium Resolution camera that's also onboard, however, should still provide images that will surpass anything taken thus far.
2) If you're struggling to observe the comet, it's about a magnitude fainter than predicted. Results announced last week by astronomers in the United States show that the comet's gas/dust output is much lower than it was at the last favourable return in 1994, which itself was also lower than at the previous favourable return in 1983.
...sounds like Deep Impact is just the thing to give the comet a little (albeit temporary?) boost!
