So, on Paul's advice, I decided to see if maybe there was a driver screwup on my Mac. Here's what I did.
First I switched the Epson inks back in and ran a few purge pages; I did a test print or two from the Mac, and they match what I got before.
Installed Eye-One Match on my PC and profiled my monitor (I didn't actually change any settings, just profiled it - the settings were still correct from my last bout of profiling, although the luminance is 114 instead of 100, which is curious).
Installed Photoshop Elements 6 on my PC (neat, the printer included a copy).
Installed the Epson driver plus other software. I carefully did not plug in the printer until the software installation told me to do so. That appeared to go well.
Fired up Photoshop Elements and loaded a test image - it's in Lab color, which Elements doesn't support, so I converted it to RGB.
I followed Epson's instructions to print it. Yes, I chose the correct printer profile in the Elements dialog, and the correct media type in the printer driver dialog.
It came out exactly the same as the prints from my Mac - dark, oversaturated.
Then I tried printing again, but with Photo RPM quality instead of Photo quality. And, once again, the print came out with blotches of wet ink on it.
Gr.