I have heard some caution that Kerry was ahead in 2004 yet lost (Ohio, I know). I agree that it is wise not to be over confident. Kerry won the first debate by consensus. However, it didn't put Kerry ahead. I checked Real Clear Politics and found the chart below. Kerry held a lead after the convention but never led in the polls after the Bush convention. Polls in late September are more meaningful than the earlier polls because most people are making up their minds now and less likely to switch.
RCP changed their format for 2008 so I can't find an exactly comparable graph. They show Obama up 4.3 % for the period of 9/21 - 9/26. Obama has led consistently except for the period after the GOP convention and the Barbie bounce. Here is a link to the 2008 data.
http://www.realclearpolitics.com/epolls/2008/president/us/general_election_mccain_vs_obama-225.html If someone can put together a good graph for 2008 please post it.