Brian, I'm not aware of published studies. An indirect way to gauge current usage of Xilinx and Altera DSP high-level design tools is to scan their online forums.
Altera - http://www.alteraforum.com/forum/forumdisplay.php?f=34
Xilinx - http://forums.xilinx.com/xlnx/board?board.id=DSPTOOL

Thanks Eric. Are there any studies done by the FPGA companies that could shed light on how their customers use high-level descriptions? It would even be nice just to know how many of them are using high-level input.

Market share data is a useful metric, but in FPGA design in particular it has its limitations. FPGA companies today offer HDL code generation products from high-level descriptions with many successful users, but their user communities aren't accounted for in the published studies.
This is similar to the situation that has existed for many years in FPGA synthesis, where published market share reports acknowledged that they could not account for Xilinx and Altera synthesis usage.

Hi Brian,

The latest market trend reports from Gary Smith EDA provides an interesting perpsective on HLS tools. As usual, the report looks at product market share, but from this list, you can easily derive trends per input language and see how C/C++, SystemC or Simulink compare.

And it gets even more interesting is you compare the 2009 numbers with 2008 and 2007. You can find the results at the bottom of this page:
http://blogs.mentor.com/thomasbollaert/blog/2009/11/17/systemc-ten-years-later…/

Cheers,
Thomas