A few notes regarding this feature:
1. Google Wallet is ONLY for phones OUTSIDE of the ATT, T-Mobile, Verizon Wireless networks. These 3 carriers have a competing product call ISIS that uses your phone's NFC for payment similar to Google Wallet and the Visa/Mastercard/Discover blink transaction concept. As a result these 3 carriers has locked Google Wallet's feature out of the phones. ISIS is only in 3 or 4 test markets now, no word on boarder rollout. I was gun-ho on using Google Wallet when I got my GN2 (on Tmobile), then found out that I couldn't use it b/c of the competing product. I'm all about these blink transactions and look forward to the day I don't have to carry my credit card around.
2. It's a common identity theft crime for theives to "bump" into your wallet or purse with a scanner that scans your credit cards that have these chips. If you don't use these features, the most common practice is to surgically remove that chip out of your card. Be careful not to mess with the standard strip that is used to swipe. You may always receive a card without a feature upon request (depending on your card issuer). Alternatively, there are also wallets and holders out there that prevent these scanners from getting through. At my work, I was issued a pass holder that does that, it's a PITA, b/c I have to take my pass out of my holder everytime to badge in to work.
3. Unless you have your NFC turned on all the time on your phone, I'm not sure how a scanner can get access to your phone. My advice is to only turn on NFC when you need it. Given the limited use of NFC products in the US, you likely will have it turned on on a as-needed basis. In other countries, they've rolled out NFC features as "fast passes" for public transportation and stuff, but same concept, turn it on only when you need it to minimize scanners out there looking for your info.