What are the risks of in-person private currency exchanging for travellers? This answer suggests popularity growth of people privately exchanging bitcoins for a physical currency, in person. This might occur in "coffee shops, hotels, hostels, hackerspaces, etc".

An example would be 2 travellers meeting in a hostel in, say, the UK. Traveller A has just been to the USA and traveller B is about to go there. Traveller A has US$200 that's no longer needed, so he agrees to exchange it for some of traveller B's bitcoins.

Definitely assume traveller B has a mobile phone or other internet access to perform the transfer, but please consider both cases of traveller A having and not having similar access at the time.

What are the risks of such a deal, and what can be done to improve the chances of a successful deal?
I think that "personal safety risks" are outside the scope of this website, so I'll mention the technical risk - double spend. If you don't wait for one confirmation, a double spent attack can be performed, and you won't get your Bitcoins.

Luckily, you'll only have to wait five ten minutes on average for a block to be found (blocks are one every ten minutes, so the expected delay until the next block is five minutes).

After one confirmation, it's very unlikely such a small transaction will be double spent. If you want to increase your security, wait another block or two.

Traveller A doesn't need to have internet, he can use a Pimpcoin-like card (link NSFW), with SMS notifications of incoming transactions.