Well having set the hare loose, I'll put it back in its form. I've used a VHM-802 PWM fan controller board measuring 19mm x 39mm that cost me less than £6 on eBay.  I've also wired a 50K ohm pot as a plug-in replacement for the thermistor supplied with the controller board to provide user speed control rather than temperature control.  Using this, I've controlled both the stock Dell rear fan and stock Dell CPU cooler fan, plus others.  I've also shown that a 3-pin 12V DC fan runs at full speed and isn't controlled by the board. I can therefore be categorical in saying that the stock Dell rear fan is definitely PWM. I've also now got a simple PWM fan controller for less than a tenner that I might deploy in a computer in the future. 
I've asked this question in a thread about improving the cooling for the XPS 8940, but would like to get responses from the wider community. One of the upgrades that people are performing on their XPS 8940 is to replace the rear fan with a 92mm PWM fan.  Many have replaced the rear panel fan with a Noctua NF-A9 PWM fan, whilst I have used an Arctic F9 PWM fan.  But are we sure that the stock Dell rear panel fan on the XPS 8940 is actually Pulse Width Modulated and that the fan control used by the XPS 8940 is PWM?  It looks to be, with 4 wires, but  looks can be deceiving. The stock fan that came with my XPS 8940 is Delta Electronics Inc. DC Brushless fan AUB0812HHD rated at 12V, 0.40A, Dell P/N D4NJ4-A00. Nowhere on the fan does it say PWM, or anything similar.  Is it possible that the two extra pins are being used to check for 'fan not present', hence why some people get a BIOS error when they remove the fan?  I have pulled up the Delta Electronics Inc. data sheet on the AUB0812 series of fans and these are voltage-controlled DC fans, although only 2-wire.  The AUB0812HHD is listed with a voltage range of 5.0 to 13.8V, 0.32A and max. speed of 4800 RPM.  Delta Electronics Inc. do not list PWM fans amongst their products. If the stock fan is DC, then this might explain why my replacement PWM fan is running at full speed all the time. Has anyone connected the XPS 8940 rear panel fan to another computer and shown it to work in PWM, or can confirm this another way, possibly using the Noctua NA-FC1?  Or confirm that the output from the Dell motherboard is providing PWM?  What are the implications of connecting a PWM fan to a non-PWM fan header?
XPS 8940, rear panel fan - definitely PWM?