...but since each dual-core shows up once instead of twice in /proc/cpuinfo, instead of getting 2, 4, or 8 actual cores I think it's 1, 2, or 4. Not sure if this means on e.g. the Large you get 1 core on each of 2 CPUs, or both cores on 1 CPU (which could matter in the case of shared cache and also for memory access).
"2 compute units" means a 2 GHz core instead of 1 GHz, not 2 separate cores. So I suppose on the Small you get half of one 2.6 GHz core.
Large instance (2 virtual cores with 2 EC2 Compute Units each):