The message from the Safaricom website is:
Get 100% bonus on ALL free minutes redeemed using Bonga Points in the Zawadi za Bonga Points promotion.
The promotion is open to all individual PrePay and PostPay customers enrolled onto the Bonga Points loyalty program valid between 11th Jan to 11th Feb 2012
So before you happily go to redeem the bonga points and choose other options like free airtime, data and other goodies, please note that the offer is only for free minutes. Lest you get disappointed.