Hellpop, the 992 bus goes from the airport to downtown in about 20 minutes. The convention center is about a 15-minute walk from the 992 bus terminal (it's a nice walk along the shoreline). Also, you could transfer from the 992 to the Orange Line Trolley right to the SDCCenter.
It's pretty simple; I've been staying in La Mesa and taking the trolley in for several years. It's great unless you're an all-night partier (but that's what taxis are for!)
I recommend taking public transportation and walking. I'll be there at the con and on the trolley. http://www.sdcommute.com/