When it comes to wear leveling, there are actually two different elements at work -- wear leveling algorithms and host usage patterns.
At the bottom level you have wear leveling algorithms on the card. The way these work is far too complicated to explain here and even varies between manufacturers. The basic idea is that in addition to the sectors you see, there are also sectors hidden from the host. For example a 512 MB SanDisk card typically has 15 MB of sectors reserved for wear-leveling. Even when you fill the card (from the filesystem's perspective), those sectors remain free and subsequent writes continue to cycle through this "erase buffer". Usually the card is broken into zones (SanDisk typically uses 4 MB zones) and each zone gets a 3% erase buffer. Repeated writes to the same zone will cycle through that zone's erase buffer but won't be able to take advantage of erase buffers in other zones.
That's where host usage patterns come in. At the top level you have the filesystem that allocates requests for additional storage (either for new files or for growing existing files) from the file system free space on the card. By keeping a decent sized chunk of free space (larger than a single zone), you natuarally increase the odds of allocations being spread around the "disk". In our SanDisk example, 10MB of free space is guaranteed to span at least three zones helping to naturally randomize writes between different erase buffers.
There's a
SanDisk Whitepaper that goes into a lot of detail on SanDisk's approach to wear leveling. It's pretty technical, but it has some good info. It's interesting to note that thier worst case example is a data logging operation using a 1GB card where a new 4kB file is written every five seconds. They factor in fixed areas of the filesystem, etc. and compute that the lifespan of the card will be 79.3 years.
Now, granted this is a SanDisk whitepaper and there may be much worse worst case examples, but I think it's safe to say that there's little justification for the high level of consumer concern over CF/SD write wear. In reality, the practical life of a memory card is probably no more than 5 years (at which point you'll want/need to upgrade to a higher capacity card) and with decent wear leveling algorithms, someone would have to really work at it to wear out a card in that timeframe.
