A friend of mine knows a customs agent and called him to see what happens when people bring back ISOMs from various trips (we happened to be returning from the Middle East). The agent said that normally they won't check, but if they do, they tend to cease all cigars because the average customs agent can't tell the difference between ISOM and other. They don't go by labels because too many people switch them. He also said that (for the most part), they just seize the cigars (probably for personal consumption to verify that they were indeed ISOMs) and don't fine people. The fines can be pretty stiff, so I'd definitely plead ignorance if they do happen to find some.
Personally, I bought a cabinet selection of Hondurans, claimed them (as Honduran cigars), kept them in my carry-on and hid the others in a traveldor in the bottom of my suitcase. The customs agent said, Honduran cigars, huh? I said yep. He said ok, welcome home! Just my experience...