I'm going to be bouncing between Thailand, Cambodia, Laos, and Vietnam for just over 2 months and I like to have a cell phone with me when I travel just in case of emergencies. When I went to the UK I bought a cheap one in London from Vodaphone that worked everywhere I went. I'm from Canada and I don't have an unlocked mobile so I would need a phone AND a SIM card. If I buy one in Bangkok (cheap junky phone that does the basics - text and call) would that seem unreasonable or pointless to anyone? Does anyone know roughly how much that would cost and if it would work to bounce between countries? Would I have to buy a separate SIM card in each country?
Not unreasonable at all... In the past I've always bought cheap but functional phones just for the purpose of being available to family in case of emergency and staying connected. You can pick up a "bare bones" cell phone in Bangkok for around 1,000 baht (this obviously wouldn't be able to run internet but would be fine for basic calling/texting). You'll want to go to MBK 4th floor to buy the phone (take the BTS skytrain, get off at National Stadium, and follow signs to MBK; if you find yourself in the Tokyu department store then find your way out and take the elevator up to 4th floor... there must be about 200+ phone shops there).
On the issue of SIM, you will unfortunately need a different card for each country. You'll be able to get the Thai SIM wherever you buy the phone in Bangkok... I think the non-internet SIM is only around 100 baht, but you'll need to add minutes (another 100-500 baht depending on how much talking you want to do). If you run short on minutes you can top up at any 7/11 or phone shop.
Cambodia is the only country of the four where I've never bought a SIM, but in Vietnam and Laos you can go into virtually any convenience store and buy a new SIM plus minutes. Obviously you'll have a different phone number for each country. I assume it works the same in Cambodia. Any questions feel free to ask.