I bought a "Manhattan Portable Power Bank" yesterday at PB Tech for about $33. They come with 3 attachments (including a USB connector) to allow you to charge up devices like smart phones and iPods. It's about the length and width of a credit/atm card and about 0.5 cm thick. 

I used it today with my HTC Sensation at NetHui in Auckland - so the phone was always on, more or less.

When the Sensation got down to about 50% (being used constantly for most of 5 hours)....I plugged the power bank in and carried on using the phone. I disconnected it when the phone charge was back up to 80%. The portable power bank was 1/3 depleted at that point. 

So it looks like (roughly), this wee device effectively gives me the equivalent of 1.5 times more battery in addition to the battery already in the phone....allowing for it charging the phone up by 30% as I was simultaneously using it for about 2 hours.

This feels like a win. No need to flick the back off the phone and re-boot. Much cheaper than buying a specific battery for this phone....and I can use it with any device that connects to 5V USB (plus the other two connectors).