After a few weeks of, it turns out that recent MacBook and MacBook Pro models - both the just-introduced aluminum-and-glass models and the two previous minor updates - can address not just 4 GB of RAM, as Apple's technical specifications pages state, but 6 GB of RAM.
(To identify if your MacBook or MacBook Pro is new enough, run System Profiler and in the Hardware Overview screen, check the Model Identifier line. After the model name are two numbers, separated by a comma, as in "3,1". If the first number is 3, 4, or 5, the Mac should be able to handle 6 GB of RAM.)
The laptops both have a pair of DIMM slots. The current MacBook and MacBook Pro models require a new form of high-speed memory called DDR3, running at 1066 MHz. The previous models use DDR2 RAM running at 667 MHz. Standard configurations have either 2 GB or 4 GB of RAM, achieved by installing a pair of either 1 GB or 2 GB DIMMs.
So what if you replaced one 2 GB DIMM with a 4 GB DIMM? The answer seems to be that the MacBook and MacBook Pro both operate reliably with 6 GB of RAM, as long as it's the same type and speed of RAM. However, reports indicate that the next logical step - installing a pair of 4 GB DIMMs for a total of 8 GB of RAM - does not work properly. As yet, it's unclear if the problem could be resolved in software (such as by Snow Leopard, the next major update to Mac OS X), or if there are hardware issues.
There are some downsides to jumping to 6 GB. First, you must install mismatched DIMM sizes (one 2 GB DIMM and one 4 GB DIMM). When working with a pair of identical DIMMs, the Mac can take advantage of its to increase the speed with which data can move from RAM to the CPU. However, for most usage patterns, a dual-channel architecture provides only a slight speed improvement, and losing that is probably outweighed by the benefit of reduced virtual memory disk swapping.
At the moment, there is another problem: price. Ramjet just announced the first 4 GB DDR3-1066 DIMM for the recently released and  models, and it's not cheap, at $599. In comparison, a 2 GB DDR3-1066 DIMM costs only $75 from Ramjet. For the previous generations of the laptops, a 4 GB DDR2-667 DIMM is a lot cheaper, at $159.99 from . Personally, I'd wait for the price to come down on the 4 GB DDR3-1066 DIMM.
And lastly, I must stress that this is an unsupported configuration, and I have not tried it personally. If you have problems and call Apple for help, they will be entirely justified in giggling at you. Don't say you weren't warned!