I know this is an old post but I was having the exact same problem. I upgraded with slackpkg 2.80.2 on my laptop and all went well, when I tried it on my desktop which had version 2.80.1 I had the same problem as xri, it kept telling me I needed to uncomment One line, which I had already done. I compared the two versions of slackpkg and found line 40 was different. I replaced it with the one that worked, version 2.80.2 and then I was able to run slackpkg update successfully. Slackware-current install was from July 28 2009.
VERSION=2.80.1 line 40
SOURCE=$(grep -e "^\([a-z]\)" $CONF/mirrors)
VERSION=2.80.2 line 40
SOURCE=$(sed -e 's/^[[:blank:]]*//' $CONF/mirrors | grep -m1 -e "^\([a-z]\)")
I also had a space before the uncommented mirror but it still worked after I changed line 40.
# USA, 45Mbit
# USA, 155Mbps
# Taiwan, 1Gbit