I am currently planning a minimum six month trip throughout Southeast Asia beginning in Feb. '09. After the first couple months in Thailand, we'll be spending May, June, July, August and September slowly making our way through Cambodia to far south Vietnam, all the way up to far north Vietnam and through northern Laos until we hope to reach northern Thailand around October. We have no time restrictions and are avoiding booked commitments.
Does anyone have advice for travelling through these places (with time but not money to spare) during the rainy months?
The countries you mention get different monsoons. See the weather section for each country (go to homepage, links under each section with relevant weather bits) for details on the conditions.
Only general advice is beaches are no good in wet season -- so looking at your rough timeline, if you're going to the Cambodian coast or say Phu Quoc Island in Vietnam in wet season, then, pack an umbrella, not suncream.
Without more info on where you'll be when, it's difficult to give good advice.
Generally speaking, travelling in wet season isn't too bad -- at least the crowds are low and the national parks etc can be very good that time of year -- roads less so.