I trust you on this, but I am surprised. I thought servers were well devices that sort of served out rather than had new stuff overwritten all the time.

Maybe with a more intelligent OS that keeps track of which segments are never overwritten and swap the data with those segments that have been overwritten half a million time already then this shortcoming could be overcome.

Think about a server that takes credit card payments for example. That's a stack load of writing right there. Imagine Amazon or another massive online retailer with the number of transactions they take, or even worse Visa / Mastercard where it's every transaction being logged.

A SQL or exchange server is constantly writing to the drives in logfile form then has to write all the logs in to the database later & delete the logs (& start over again)

Depends on the server I guess. A generic media server would spend it's time reading and serving movie files and audio files, etc....Whereas a server that provisions virtual machines for general use over a high-speed network could see a significantly higher rate of writes (and depending on the OS virtualised and the Virtual Machine software used, lots of overwrites I guess)

I would expect to see them used en masse in a investment bank. These would be useful for high frequency trading.

Surely far too much writing every time the prices update? X thousand companies updating every 30 seconds or whatever is going to be pretty savage on an SSD.

Yes but banks can replace them every few months if necessary they at least make enough money to justify such an expense. Few others can.

There are consumer and enterprise rated SSDs, they are normally rated for their "write endurance", rather than MTBF.

The consumer ones (usually multi-level cells as they are cheaper to produce), have a "lifetime" of approx 20 to 40TB of data writing, whereas enterprise rated SSDs (ideal for servers, and based on single-level cells) can be rated up to 2 (PB) Petabytes of data writing (1000 TB). See the Intel 64GB X25-E for info.

When prices for single-level cells drop they'll be wide-spread.

Surely at around that amount of writing a traditional HD is likely to fail as well.

I'd be tempted to have a small 40GB or so in my next desktop PC as the OS boot drive, and then a large normal drive for data. In a laptop I can see a real benefit too.

You should see the fusionIO drives. They're designed for that sort of thing. They can stream over 140 1080p films from one without it stuttering.
They also have spare chips built in that can be used if cells start breaking down.