As one of the few players that actually main Rammus, I can answer that one:
The key to beating Rammus is to either 1.) focus him when his DBC is on Cooldown (you'll know it), 2.) focus his squishy teammates, or 3.) Build MRes/Tenacity. Rammus isn't really strong unless he has a strong team behind him, coupled with the inexperience on how to handle him. He's really vulnerable to AP champions even though he gets MRes/Level, and the bad thing about Rammus is that should he be focused he dies pretty fast.
In Season 2, Rammus was definitely an issue. Now he's more of a POOT-ing champ due to how much armor has been devalued in comparison to the Armor Penetration options. To counter a Rammus, after successfully grabbing top, have at least one champ guarding the speed shrine near your point or the one near theirs, since he will likely POOT to mid, up to top if his team does not have top, or attempt to gank bot.. When encountering him, either pick a squishy target to focus or simply focus him. Once either his team dies around him or he dies, it's really easy to take his entire team down. Should he go bot, just focus the guy he's trying to help. It's really easy to deal with him now.