Joseph Browning post a simple work around for small parties - just use an adventure that is designed for a lower level party. For example, a party composed of two 5th level characters will probably get a decent challenge from an adventure written for a 3rd level part of 4 to 6 characters. It's something you need to eyeball more that have a chart or equation to check the balance, but it's doable.
The question then becomes - what of small lower level parties? Two 1st level characters won't last long in the average adventure designed for 4 to 6 1st level characters - that's a fact, especially if it's combat heavy.
Yes, I know -"I write adventures specific to my player characters abilities". Regretfully, not all of us have that amount of free time (and others lack the skill or inclination). That is why adventures are such a popular purchase, especially on RPGNow.
Joseph also posted today that his most popular adventures by sales numbers are the low level adventures.
By extension, this means, at least in my mind, that low level adventures for smaller parties have an underserved audience.