Yes, you can install on multiple devices apparently despite it saying only 1 activated device per license. I installed mywi (bought yesterday) on both my i4 and my iPad, both of which have been linked to my google account in cydia via their unique UDID's.
In other words, it will work if you own both devices. If it's someone else's device that has already had UDID paired with a different google or facebook account through cydia, it might not work. I wouldn't know because I haven't installed it on a device that isn't linked to my google account on cydia.
People, the app is worth the $$ and you know it. Your data alone on the phone costs more than that in 1 month, voice and text aside. I've been getting by fine with the $2 tether me cydia app and Bluetooth is really nice and efficient for tethering just to my mbp but it's so nice to have the flexibility of a wifi hot spot. I've run meetings on mywi before (had trial a while back) and it worked wonderfully. The only thing that kept me from buying it before was that it depended on rock which I never liked and when I read saurik's recent criticism about it's shortcomings I think I made the right choice by not buying it until it was licensed through cydia. As soon as it no longer carried rock app as a dependency I bought it.
Also, iPhone 4 users out there, this app is especially worth the $$ because of how fast the iPhone 4 data rates get. I'm pretty steadily at 3 megabit down or so and slightly over 1 megabit up. Using the web while tethered with mywi feels just like broadband did 7 years ago. I know we are all spoiled by our 20+ megabit comcast connections and what not but fir something that works (pretty much) everywhere I go, pretty amazing.