I doublechecked - I believe it's only the 5580 and 5585 (and possibly 5545-X and 5555-X). that display it in ASDM. It doesn't specifically mention SNMP but see the reference here where the information is described as available on those platforms.
Also see here where the "show environment" command description states:
You can display operating environment information on the ASA 5545-X, 5555-X, 5580 and 5585-X. This information includes the operational status of the fans and power supplies, and temperature and status of the CPUs and chassis.
The same platforms are listed in the 9.1 command reference. However I just checked on a 5525X running 9.1 and see:
# sh inv | i Chassis
Name: "Chassis", DESCR: "ASA 5525-X with SW, 8 GE Data, 1 GE Mgmt, AC"
# sh env temp
Temperature:
-----------------------------------
Processors:
--------------------------------
Processor 1: 63.0 C - OK
Chassis:
--------------------------------
Ambient 1: 43.0 C - OK (Chassis Back Temperature)
Ambient 2: 33.0 C - OK (Chassis Front Temperature)
Ambient 3: 41.0 C - OK (Chassis Back Left Temperature)
So I think it is all of the X series as I originally replied. That includes being able to see the temperatures in both ASDM and from command line.
I did check a 5550 (running 8.4) and the "show environment" command is not supported.
# show inv | i Chassis
Name: "Chassis", DESCR: "ASA 5550 Adaptive Security Appliance"
# sh env
^
ERROR: % Invalid input detected at '^' marker.
