Let's face it, pretty soon every airline will charge for a second checked bag, and then they'll probably all raise the fee.
JetBlue will begin charging $20 for a second checked bag for passengers traveling after June 1. No word if this fee is retroactive to tickets purchaed before then for travel after the effective date, which probably means you won't be grandfathered. Since JB doesn't have "elite" frequent flyer members, or first class, looks like there are no exemptions as there are with other airlines.
Air Canada will be charging $25 (is that US or Canadian? We're not sure, but the two currencies are about the same now) for a second checked bag, but only for Tango and Tango Plus class flyers. Travelers purchasing more expensive Latitude and business class tickets are not affected. The new policy kicks in on July 15, and again, no word on whether tickets bought before then will be affected.