A month in the US
 

My son is planning a one month trip to the USA - a week in NYC and three weeks in California, Nevada and Arizona.
For communication with home he will use either an IL HOT-mobile or IL Golan telecom SIM, both offering very cheap international roaming, competing with international calls from a US mobile number. However, he needs a prepaid data SIM (or plan).
Which is the preferred carrier and plan for his needs, considering that he will use an unlocked AT&T Nexus S handset?

You need to go with a ATT or one of the virtual networks which use their network. The ATT variant of the Nexus S doesn't support TMobile's 3g/"4g" frequencies.

In my opinion, the best deal is the Straight Talk SIM from Walmart which is $45 for unlimited everything:

Straight Talk SIM Card + iPhone 4S = $45 Unlimited Prepaid Plan My Money Blog

There are people that say there is a softcap on it. Pocketmobile also uses ATT's network. I'm not sure how cheap your roaming is on the Israeli providers, but I would think you'd come out cheaper with a virtual Israeli number forwarded to the US number and a cheap calling card back to Israel. I like VOIP Cheap (a Betamax company) with the Mobile VOIP app. 300 minutes a week are free to landlines, it supports VOIP over 3g, Callback, and dial through. 10 euros gets you 90 days worth of coverage.

Let me add (before someone else does) TMobile is rolling out HSPA+ on the 1900mhz band, but it is hardly universal. Using TMobile, your son will find some 3g, but not everywhere.

Thanks very much.

The new Israeli operators offer very attractive roaming rates: HOT mobile charges ILS 1(=USD 0.25/EUR 0.20)/min for incoming and callback to Israel and ILS 0.39 (USD 1/EUR 0.8)/min for local and intarnational calls from Eurpoe, USA and Canada. Golan is expected to be cheaper from Eorope when they will start international roaming later in July.

They are excellent rates and I am impressed. For the record, VOIP.MS offers an Israeli landline number for US$9 a month and will forward to US phone numbers for under one US cent a minute. Mydivert.com has Israeli numbers for €4,50 per month. Forwarding to US numbers is around 1 cent a minute. Your solution may make more sense (depending on usage).