I have recently used jailbreakme.com to make my iPhone 4 free. However, fairly soon, I've discovered that I have a significant battery drain with it. It took literally overnight to drop my 25% from fully charged one. So I decided to took few tries and find out who's responsible for this leak. On that particular jailbroken version, it was going down to 87% with 30 mins of usage and 3 hr 42 mins of standby time. 
The clean restored iPhone stays for 32 mins of usage and 4 hr 47 mins of standby, and goes down to 95% only! That's more than 3 times. Not good. Next, going thru the jailbreak, not touching Cydia or any apps. iPhone stays for 17 min of usage and 3 hr 35 mins and down to 96% only. Basically, that's a sign that jailbreak indeed doesn't drain the battery. Next, installing all of the updates that Cydia wants you to. It took me overnight to measure it, but seeing 15 min of usage and 9 hr 33 mins of standby and down to 95% only is clearly a good indicator of no battery drain going on. The last step was to install MyWi 4.3.4. It comes also with Mobile Subtrate. After spending some time with it - 1 hr 17 mins of usage and 5 hr 45 mins standby - brought my phone down to 78%, which is definitely stands out of the line of "good battery usage". So, either Mobile Subtrate is draining the battery or MyWi didn't do a good job writing an extension to it, and now it eats all he juice. I'll keep you updated, but I'm already upset so much - the most important reason for me was free tethering - and now I feel that I'm paying too big premium for it.
The clean restored iPhone stays for 32 mins of usage and 4 hr 47 mins of standby, and goes down to 95% only! That's more than 3 times. Not good. Next, going thru the jailbreak, not touching Cydia or any apps. iPhone stays for 17 min of usage and 3 hr 35 mins and down to 96% only. Basically, that's a sign that jailbreak indeed doesn't drain the battery. Next, installing all of the updates that Cydia wants you to. It took me overnight to measure it, but seeing 15 min of usage and 9 hr 33 mins of standby and down to 95% only is clearly a good indicator of no battery drain going on. The last step was to install MyWi 4.3.4. It comes also with Mobile Subtrate. After spending some time with it - 1 hr 17 mins of usage and 5 hr 45 mins standby - brought my phone down to 78%, which is definitely stands out of the line of "good battery usage". So, either Mobile Subtrate is draining the battery or MyWi didn't do a good job writing an extension to it, and now it eats all he juice. I'll keep you updated, but I'm already upset so much - the most important reason for me was free tethering - and now I feel that I'm paying too big premium for it. 