I've got hard data for Visual C# 2008. The short version is that you are best of spending your money on a faster CPU than faster IO. Longer answer follows...
Our C# (.NET 3.5) solution contains 81 projects with over 2M lines of code (including comments and blank lines). A couple of years ago we upgraded from Pentium 4 3GHz PCs with standard HDDs to Core 2 Duo 2.6GHz PCs with 10,000RPM WD Raptor HDDs (74GB). The speedup was immense. About 10 minutes down to 3.5 minutes. All of this in in a Windows XP Pro 32-bit environment with 4GB of RAM.
We also got one Gigabyte i-RAM (google it for info), which is basically a RAM hard disk with battery backup. Unlike an SSD which is fast for reading but slower for writing, the i-RAM is fast for both, but if you lose power then the battery only lasts for about 12 hours so you have to be disciplined with your check ins. This shaved another minute off the compile times on the Core 2 Dou platform (down to 2.5 minutes) compared to the 10,000 RPM Raptor HDD.
I've since discovered that those old 74GB 10,000 RPM Raptor drives are slightly slower than your garden variety 7,200 RPM modern drive and we've proven that consistently benchmarking compiles. We haven't tried the new Velociraptors but they would certainly be quicker but probably not enough to be worth it for compile times alone.
Last week we got a new Intel Core i7-870 platform with a G.Skill Falcon 128GB SSD (with the Indilix Barefoot controller) and a standard 500GB HDD as the second drive. I also chucked the i-RAM into this PC and tested all configurations.
Compared to the Core 2 Duo, which compiled in 3.5 minutes for HDD and 2.5 minutes for the i-RAM, the i7-870 compiles in 1 min 40 seconds for the SSD, HDD and i-RAM give or take 3 seconds.
So both times we've upgraded developer workstations, the vast majority of performance improvement in C# compile times has come from faster the CPU rather than faster disk. If you want to speed up compile times, put your money into the CPU rather than the disk.
That said, the SSD is much faster for loading VS and opening a solution (although I haven't got timings for that). If you can afford an SSD you'll never go back as every program on your PC loads so much faster it is incredible. But it won't significantly speed up your compiles. And that's with VS C# being single-threaded. If MS ever got their act together and made their compiler in the IDE multi-threaded then we could actually use those 4 cores...