There are third-party apps that add speed-dial capabilities, including a few free ones, but I'm partial to the DIY option presented in the above video. It's pretty straightforward: All you do is type the person's number into the Safari address bar, tack on .tel.qlnk.net, and then tap Go. (Watch the vid for complete instructions.)
My sole complaint is that when you tap the newly created speed-dial icon, you then have to tap the Call button that pops up. So technically it's a two-tap speed dial. But it's still very handy.