Mainly Siri, but there were a few other mitigating factors:
1) My wife had a 3Gs, and doesn't care for the latest and greatest. Since she will pretty much always be OK with the non-latest model, we could upgrade and not worry about losing upgrade eligibility. I used my upgrade for the 4S. My wife got my 4, sold the 3Gs to pay for the upgrade. When the 5 (or whatever they call it) comes out, I'll use my wife's upgrade, get the 5, and she'll get the 4S. Sell the 4 to pay for the upgrade. Repeat for every new release. 🙂
2) I had been very close to full on my 16GB iPhone 4 for some time. Going to the 32 GB 4S has been nice. 🙂
3) I'm a photographer. While I rarely shoot with a compact or compact type camera, always carrying my big camera kit, I had been using my iPhone 4 quite a bit for casual snaps and trips/outings where photography wasn't the motivation. Having a higher quality camera is always a plus. This has been a +/- thing after getting it. The 4S camera is very good within 10 feet in good light, but it's pretty bad for landscape shooting (distant subjects are horribly mushy and noisy).