Jumped into my 99MY this morning only to find that it had developed a hard brake pedal with no slowing ability! Judicious pumping off the pedal brought them back to life. Dealer didn't know what it could be, but I have experienced similar with my last car BMW 328 Coupe and some cars back, a Cavalier 4x4 Turbo. Anyone else experienced similar?

Yes,

once a long time ago but not in a Subaru. It turned out to be the floor mat, which had slid forward enough to get wedged under the pedal.

Probably not what you had, but it certainly gave me a fright when it happened!

Sounds like it could be something to do with the ABS?

Difficult to know from your description but it could be a defective servo. The consequence would be that greatly increased pedal pressure would be necessary for the same level of braking.

I've noticed that if it's been cold and wet, the brakes don't work to begin with. Slightly alarming to say the least... Anyway, foot hard down eventually gets them to bite, then it's business as usual.