Hi bluefront,
That's a really rare request. We used to sell such fans but they were selling really bad. The VR fans are selling better but nothing compared to normal "fixed" RPM fans.
The fans with a variable range of RPM are usually more nosie, because thesy can not be optimized for high or low rpm, this casues extra noise. There is nothing like thermal sensor controlled fan in development and I assume this might not change. PWM is kind of doing a similar job for CPU Coolers, doesn't it?
Cheers