So how common is wifi/internet access in SE Asia. And more specifically for: Cambodia, Vietnam, Laos, Thailand and Malaysia?
Like Jonik said, the WIFI usually only covers hotels, cafes and malls (shopping centres).
In Singapore however, the coverage is bigger, you can get WIFI freely across the island using Wireless@SG. The registration and usage are free. However the bandwidth really depends on the location.
In Malaysia / Indonesia, it would be better to buy a temporary 3G plan.
In Malaysia , you can use Maxis / Digi and it will give you unlimited internet access for 5 days. The price is around 24 Ringgit or around 8 USD ( in early 2012 ).
In Indonesia, you can use Telkomsel Flash. The price for 1 month unlimited internet access is around 30.000-40.000 IDR ( 3- 4 USD ). However the sim card sold separately - which will cost you 20.000 IDR or around 2 USD ( price is valid for as per as end 2012 ).
You should have no problem in any of the countries you listed. As others have said, places like Singapore, Malaysia, Vietnam and Thailand will have more reliable internet/wife access. But even in Cambodia and Loas, which are less developed, you should have no problem. As long as you are in more heavily touristed areas (and not the rural countryside) you will be able to find many guesthouses, cafes, and restaurants with wifi or computers you can pay to use. There are also internet cafes in most major cities where you can pay to use the internet for very cheap.
If you're going to stay in hostels and guest houses where the hosts speak English (especially in Vietnam, Thailand, and Malaysia), you probably won't have trouble finding internet cafes in most villages.
If you're going further off the beaten path, don't expect much. Even larger towns in Laos will sometimes have random power outages. I can't speak for Cambodia but I expect it's even harder to find there.
As for wifi, I wouldn't expect it in too many places: fewer people own computers, so while internet cafes are very popular, wifi is generally sparse. If you're considering bringing a laptop, I'd advise against it.