The contractor building my house has arranged all the rebar in the load-bearing columns and beams very close to the surface of the concrete. I've read elsewhere there should be a MINIMUM of 3/4" coverage (1 1/2" recommended). In most places coverage meets that minimum (just barely), but there are several locations where coverage measures only 1/2". Should I be concerned?

Two inches was minimum cover of concrete over reinforcing steel for structures in bridge construction, in my experience.
What do the plans and the governing building specifications require? Resistance to rusting which can result in concrete staining is one concern. Maybe there is a special reason for your particular application or maybe the steel was fabricated incorrectly by the supplier. Talk to the contractor and ask them what spec they are using.

Two inches was minimum cover of concrete over reinforcing steel for structures in bridge construction, in my experience.
What do the plans and the governing building specifications require? Resistance to rusting which can result in concrete staining is one concern. Maybe there is a special reason for your particular application or maybe the steel was fabricated incorrectly by the supplier. Talk to the contractor and ask them what spec they are using.

The plans do not specify. There are no relevant regulations, at least none that apply--the building is in Thailand, not the U.S. It's a residential single story building.

Rust is precisely my concern, and it's not staining I'm worried about, it's structural integrity. My understanding is that without adequate coverage, water reaches the concrete via capillary action through micro-cracks. This will cause the rebar to rust, which will then expand further cracking the concrete, until the concrete actually separates from the rebar.

You are 100% correct they will rust away. Smearcase is correct the minimum is 2 inches since 1995, 1 1/2" is old construction standard pre 80's I have never heard of 3/4" coverage much less 1/2" you should be concerned don't let it happen your contractor is wrong. The concrete will not stop water infiltration at less than 2" unless a polycoat (rubberized clear coat) is applied every year and still at 3/4" this just won't work as the concrete will flake off as the rebar expands.

You are 100% correct they will rust away. Smearcase is correct the minimum is 2 inches since 1995, 1 1/2" is old construction standard pre 80's I have never heard of 3/4" coverage much less 1/2" you should be concerned don't let it happen your contractor is wrong. The concrete will not stop water infiltration at less than 2" unless a polycoat (rubberized clear coat) is applied every year and still at 3/4" this just wont work as the concrete will flake off as the rebar expands.

That's what I was afraid of. Thanks for the clarification.