Nexus One's 3.5mm headphone jack accepts standard 3.5mm plug? As you know, the Nexus One comes with a 4-segment 3.5mm headphone jack, whereas the standard 3.5mm plug has 3 segments.

I visually compared a standard 3-segment 3.5mm plug with the 4-segment 3.5mm plug that comes with the original Nexus One headset and they seem identical in length.

My only concern is that plugging in a standard 3-segment into this 4-segment jack will damage the Nexus One.

Better safe than sorry... so instead of empirically plugging in the standard 3-segment plug in the the N1 just to see whether it causes a fatal short or not, I prefer to ask those who are already knowledgeable with this: Is it safe to use a standard 3-segment with the N1 headphone jack?
Your phone should be fine.

Android won't route phone calls through headphones though, iPhone can be set to do so and uses the phone's microphone.

An option I use to attach standard headphone to my Galaxy S is a simple Bluetooth headphone adapter (£10 from Amazon), that way I get to use MY headphones and make calls via them, plus you can keep your phone in your pocket when skating into town!