Connecting the occasional tiny load across one batter may be ok but if you want significant power then there is a real problem.

If you want to do a lot of this sort of thing then a separate 12 v battery with a means of charging it from the 48v is probably the only sensible way except for smaller loads that can be run directly from a 48/12v dc converter. These converters and also the charging equipment to charge 12v efficiently from 48v will be expensive. Far better to avoid these loads if at all possible.

For 24v you may be able to find buck/boost converters that will maintain the centre voltage and let you take a supply from the centre point but not a simple solution and in the long run you would need to change over the bakks to equalise the wear even if you can maintain correct charging and voltage.

What if he wants to charge his cell phone or cordless drill?
Ever look for a small 24V or 48V inverter from a name brand company?
Maybe 24V or 48V CFLs?  Switches?  

Might save $100 on wire, but in a small system, it gets lost on the other parts.
ie: Mail order a $30 switch instead of a 99 cent switch from the auto parts store.

Run a big 24V or 48V inverter to send 2W to the cell phone, and 11W to a CFL?

Depends on how it will be used.

How much power (current) do you need at 12 V ?

If it's just 2-3A max, you might have a look at DC-DC converters as shown in the picture below. They're found in old Telecom equipment (which uses -48 Vdc) where they were used to convert -48 Vdc to +/- 12V and 5V. I have a few (they're not for sale) taken out of old equipment  from a phone switching system. They can give up to 30W output at 12V, depending on the kind.

So, if you have only small 12V loads you might look into this. I'm sure on E-bay such modules are offered for sale. Farnell sells them too, if you're willing to pay Farnell prices...

http://uk.farnell.com/jsp/search/browse.jsp;jsessionid=WPQ40QJDC4UURQFIAGJZK0Q?N=500004+1001507+1278 87&Ntk=gensearch_001&Ntt=dc+dc+converter&Ntx=&_requestid=217264

Good luck,

The solution to this trouble is a battery equalizer or in more technical terms a bi-directional dc-dc converter.

There are two companies I know of that produce them and that I have product experience with... there are some others but I have no track record for those and thus can not give opinion one way or the other.

http://www.vanner.com

http://www.solarconverters.com

Vanner makes a VERY robust product sold under the name VoltMaster, but may not have your desired input output voltages.
SolarConverters products work fine and they take custom orders.. e.g. your preferred input output voltages.

Ether way usually $200 to $400 is the cost range for these products.

Also check in my files I've put together a PDF of an old article in Homepower 7 on DC distribution written by Windy Dankoff. It details the use of the Vanner and some basic informal standards that myself and other RE installers have been using for a while for  the distribution of LV-DC.

Sorry no schematic for a home brew version of the converter.. I've tried to bang neurons with some other folks over the years to develops an open source design for the diy-er but I've not caught an interested party with the required skill set to help develop and R&D one.