Although the Atrix will actually be MORE efficient at certain mundane tasks..the dual cores will SUCK it dry when its running multi-threaded games etc. Like wise the Samsung infuse is essentially OVERCLOCKED and even a "little" over clock and be a HUGE SUCKING DRAIN on the battery.
It is highly likely the inspire can/would beat either of these two devices battery's life IF they are doing intensive tasks where the extra power is used.
In the case of the inspire..users will just have to accept hey wont be running those few games or will be running them slower (but without the surging power drain)
Likewise when standing by, the Atrix use will more power than the inspire. (It doesnt appear to be nearly as efficient in idle mode as i would like to see).
Full load= Inspire over atrix
Idle load= Inspire over Atrix
Mundane tasks = atrix over inspire (the tegra can be very efficient at medium tasks).
Atrix Stand by time: 264 Hours
Inspire Stand by time: 372 Hours.
Also i am guessing that huge 4.3inch screen being in off mode is a huge power boom for the smaller battery powered inspire in addition to having a MUCH more efficient stand by power usage.
Through out my day i might be screwing with the phone a couple hours over all but its in stand by most of the time. On weekends or late at night when surfing around ill probably suck the phone dry pretty fast.
However, one of the MOST intensive tasks...GPS will run VERY EFFICIENTLY on the inspire as it preloads all the maps (avoids data charges and will SAVE THE BATTERYs big time)..so that will help as well.
HTC Inspire 4G vs Samsung Infuse 4G vs Motorola ATRIX 4G: specs comparison - Phone Arena
Also coming from an iphone background you have NO IDEA...how simple a fix it is just to buy a back up batter (something you CAN NOT DO) with an iphone. (although i LOVE LOVE LOVE the charge cases such as the mophie juice pack).