Will using a temporary travel SIM affect anything on my iPhone? I'm off to Japan for a couple of weeks and wanting to take my iPhone 4 as I need to contact a few people while I'm there.

If I use an international SIM to save massive roaming charges will it affect anything on my phone? i.e the apps or notes or anything else? I'll be putting my normal sim back in after 2 weeks.
No, it shouldn't.
Your apps and data are stored on the phone, and changing SIM card won't affect them.