Hi PC2001,
Just came back from a shopping trip to New York myself - was scared witless by all the stories about being caught by Customs on re-entry - spent the night, before we left, taking off all the tags from the stuff we bought (including the new suitcase!)
We had a big bag each and I thought we looked likely candidates for stopping - however, on checking in in JFK there were people with 3,4 and even 5 bags each !!! - One elderly lady even asked us to check in some of her bags because she was "terribly over loaded" - I didn't in case anyone is wondering.
When we arrived in Dublin there was nobody manning the customs channels - I couldn't even see anybody on duty in the "Something to Declare - Red Channel" (that was at 08:30 in the morning) so we got through ok - I have heard the same thing from other people that have gone to the States - nobody in Customs on their return.
With regard to tax back from the States, I don't believe it's possible as there is not a standard VAT rate in the states but rather each State levies it's own sales taxes.
BTW I would recommend Woodbury Common in the Catskills in New York for shopping - fantastic bargains on designer gear - there is also a Samsonite store there with excellent bargains. There were more Irish in the place than Americans the day we were there.
Have fun
efm