I'm a Canon shooter, but I'm considering buying a Nikon SB-28 as a wireless slave flash for use with PocketWizards. Without knowing too much about the Nikon system, is this a good entry point for a relatively cheap flash, high guide number, that works well with PWs? Are there other discontinued or in-production models that I should consider as well?

I've been using Canon flashes (430EX and 580EX) and have found them to not be as reliable as I'd like over longer distances with the PocketWizards. After troubleshooting with Mamiya/PocketWizard they have told me that it is a known issue that Canon flashes emit a high amount of interference, which affects the PocketWizard range negatively. I figure that if I acquire a non-Canon flash that would overcome that and allow me to trigger reliably (e.g. across a banquet hall at a wedding).

Yes, this Nikon flash is great for off camera shooting. The same is true for SB 24, 25, 26 flashes. And take advantage of "Screwlock PC" connector used in Nikon flashes and buy this to connect to you PWs:
http://www.flashzebra.com/screwlock_pc_fivemeter/0043.shtml

For off camera you can do the sb-28, the SBs that PCreating mentioned: SB-24,25,26, also Vivitar 283s and 285s. the only draw back to the vivis is the special cable you need to purchase for vivi to PW. They can all be had for $75 and under.

Thanks guys. Just bought a SB-28. Can't wait to give it a try.