Yes of course. I've used it all over the world, we can skip the basic questions and get to the good stuff ;) So you didn't even get a sim card in the end?

Just thought I'd chip in (though this is documented ad nauseam on this site and elsewhere on the web) and mention that your iPhone will work on China mobile, but for edge only. If you want 3G you'll need to go Unicom, but if you're happy with Mobile's slow edge crawl, they tend to have better coverage in the Chinese countryside. Mobile does have 3G, but it only works on network-specific phones they sell. Let us all now raise our arms and thank the heavens for China Mobile's "homegrown" 3G standard. No idea about Unicom 3G pricing, but it will probably be a lot cheaper to get a package. I think they might even have pricey unlimited data packages.

Not to mention in my original question ;)

"I will be using an American iPhone 3GS so it (from what I can tell) will only be compatible with China Unicom."

This is the pricing page for the data only plans by Unicom: 3g.10010.com/3gindex/card_money.html

However, when I got my data plan with them over a year ago, I tried to get the half year or year card in Kunming (option four at the bottom). First I was told they would only sell it at their head office (next to Panlongjiang, just south of Renmin Lu IIRC), but they could/would not sell it to me either. So I went for option 3, a monthly plan at Y80 for 1GB nationwide (plus 2GB per month free inside Yunnan only, not mentioned on the page), which has the option of using much more at decreasing cost. But you have to plonk down 1200 as initial deposit. This is a data plan (but SMS works, as does Skype and tethering).

If they sell it, the half-year card for Y300 for 3GB would probably your best option.

They needed my passport and police registration, not sure if hotel registration would do.

What is a police registration? Yikes! I'm hoping I can get this SIM either in Xi'an or Chengdu, or in the Beijing airport. Those are really my only options.

I agree that having 3 GB would be excellent. $50 is a lot but 1 GB may end up being disappointingly little. That page is very chinese... but it seems like they have a 1 month (or maybe it's "monthly" on a contract?) option with 3GB for 150 yuan... I just hope I can find somewhere in Xian to make it happen. Both there and Chengdu I have about a day in before I really hit the road.

@Blade regarding your question about which networks are best for Laos, the answer is: not the Chinese ones. I don't think Chinese carriers offer roaming (my one certainly doesn't) and even if they do, the charges will be incredibly high.

Your best bet is to buy a Lao SIM card in Huay Xai, recharge it and use that. Try Tigo Lao, Beeline (probably the cheapest network) or M phone 3G (I use that one). Alternatively, recharge your Thai SIM and pay the exorbitant roaming charges, but since there is no reason for you not to purchase a Lao SIM, do so. Even if you catch the direct Chiang Mai-Luang Prabang bus, getting off in Luang Nam Tha to catch your onward connection to Kunming via Boten, Mengla or Jinghong (Chiang Rung), the bus will still stop long enough at rest stops along the way where you can surely purchase a Lao SIM card. If you catch any bus starting from Huay Xai, you'll have even less of an excuse not to purchase a SIM card there.

Sorry, I think I misread your question...you asked about the best networks for your journey to the Lao border from Kunming. I am assuming that you also want to catch up on emails etc. in Laos so follow my advice as indicated above for Laos.

In terms of rural Yunnan, I don't think there should be a problem with coverage for any of the major carriers provided you stay to the main roads (which you will).