Well...it really depends on HOW SICK you are...of course if you work out at a gym...and you have the flu...the other patrons aren't gonna appreciate getting flu germs of course
If you're coughing and hacking up stuff, I'd say skip the gym altogether (this comes from personal experience...I had a cold and flu a few years ago and against my doctor's advice, went to the gym and worked out anyway - ended up getting larengytis (sp) and had to stay out of the gym for nearly TWO WEEKS as a result (and even worse - take antibiotics which I hate!).
If you just have the sniffles, you should be okay. Remember that if you're sick, your body will need to devote itself to 'evicting' the virus or bug you've got - I wouldn't lift weights at all if I was really sick because IMO asking your body to fight the bug AND repair muscle and recuperate from your workout is asking WAY too much.
My advice - play it by ear or if in doubt, call your doc