I live and work in the UK, but I am in the US for 2 weeks, and was wondering if I buy the new iPad from the Apple Store here in the States, will I be able to replace the AT&T or Verizon SIM with an O2 or T-mobile (UK mobile carriers) SIM? Or are the iPads sold by the Apple Store locked to US based carriers?
If you bought the device without a contract, it's unlocked!
The information about AT&T and Verizon is important when talking about LTE availability in the US. Only AT&T and Verizon provide LTE networks in the US that work with the iPad.
4G LTE is supported only on AT&T and Verizon networks in the U.S. and on Bell, Rogers, and Telus networks in Canada. footnote no.3 - iPad specs
This is important to note as LTE frequency bands used in Europe, Australia and probably most of Asia are not supported by the iPad. You will use DC-HSDPA, HSPA+ or UMTS instead for fast internet access.
Yes the iPads are unlocked with respect to international (non US sim cards) so you are not tied to a contract and free to use whatever carrier you choose internationally.
It's not clear why Apple is mentioning that UK iPads will work only with AT&T LTE here. The 3G networks between VZW and AT&T are even less compatible than their LTE implementations, but if you went with the AT&T model here, I would be extremely surprised if it's not identical to the ones sold in the UK with exception of the initial carrier settings loaded on the device (and that it will always be known to Apple as an AT&T iPad.)
You can see that the iPads sold in the UK explicitly call out that they will work with AT&T's LTE only and not with Verizon's LTE.
This could simply be a agreement or modem driver delivery choice rather than something physically different with the models. Again, until someone tears apart the various models and posts the differences, we can only go on the Apple marketing information calling out which carriers work with which other ones across the pond. I could see the verizon one being special to take better advantage of verizon's 3G (and slower) network footprint here in the US so you might avoid that one until more is known for sure.
Again, those of us buying the Verizon iPad should get coverage in europe, but perhaps not as good as the AT&T or the UK specific models.
I visited the UK from the US with a new iPad 3 which had an ATT sim card.
Upon arrival I purchased My 3 sim card from the Apple store in Lincoln (UK)and replaced the ATT card. It only cost 10 pounds for 1Gb of use for 1 month, ample for my 3 week visit.
I have had no problem connecting all over the UK and would highly recommend the My 3 sim card which was 25% of the cost of ATT in the US for one month's contract service ($15 for 250Mb per month)
The disclaimers on the Apple site (both the US and UK Apple websites) indicate the following:
The iPad with Wi-Fi + 4G model you purchase is configured to work with a particular cellular network technology. Check with your carrier for compatibility and cellular data plan availability.
4G LTE is supported only on AT&T and Verizon networks in the U.S. and on Bell, Rogers, and Telus networks in Canada. See your carrier for details.
Given that Apple keeps concluding with this recommendation, I would suggest that the safest answer is to check with your carrier in the U.K. whether an iPad from the United States would work with one of their SIMs. The documentation specifically states that 4G LTE will not work outside the specified carriers in the U.S. and Canada, but U.K. carriers do sell plans for the iPad (3rd generation).
Taken directly from the Apple site:
Both the ATT and Verizon model of iPad can roam worldwide on GSM/UMTS networks. In countries without compatible 4G LTE networks, the new iPad will operate on GSM network technologies such as HSPA+ and DC-HSDPA. When you travel internationally, you can use a micro-SIM card from a local carrier.
This means the new iPad (3) is unlocked at time of purchase. No further work necessary, I assume. This also means that you can buy the new iPad (3) from anywhere and it will work anywhere. The fact that your carrier said it be better to buy in the UK is simply because they hope you'll buy it from them. It does not matter where you buy it.
I bought my iPad 2 in the States (it is an AT&T model) and I've been using it in England since last June. Works fine on Vodafone UK. But as a thought, after paying sales tax in the US, you don't save terribly much over buying the device in the UK. The days of bargain American pricing have gone, Apple seems much more reasonable about currency differences now.
Of course, if you purchase in a no sales tax state like New Hampsire you can avoid all taxes.
All cellular capable iPads are unlocked regardless of the carrier. See this answer on Apple's Forums.