For most things, I'll need a microcomputer core on the FPGA. Xilinx has the Microblaze and Picoblaze synthesizable cores, but I want something that's open-source. That way I can see how things are done and modify as desired.
There are several open cores available. I settled on the AX8 from opencores.org. It's fairly simple, but it implements most of the Atmel AVR instruction set (which I have a lot of experience with). A simple "Hello World" in this core turned out to be pretty straightforward.
After downloading AX8 from opencores, I loaded A90S1200.vhd, AX8.vhd, AX_TC8.vhd, and AX_Port.vhd into ISE WEBPACK. Then I modified A90S1200.vhd to divide the clock down to a lower frequency and to invert the reset line (since the core is set up for an active-low reset and most of the buttons on the board are active-high). I then created a .ucf file to attach port D to the LED's.
The test program just writes to port D to blink the LED's. I assemble the program with tavrasm, then convert the program to a rom vhdl file using the hex2rom program that's included with AX8.
And that's it!. Build in ISE, download to the FPGA, and the LED's blink.
I can post source if anybody's interested.