Although I agree with the fact Bitcoin cannot be compared with the Tulip Mania, some of your argument are not correct. You take the comparison literally, while the comparison is on a more abstract level.

Bitcoin is divisible, a tulip is not

This is a non-argument, lets say we talk in Satosi instead of Bitcoin. 1 Satosi is not divisible, while a Bitcoin is.

Tulips are not scarce

Do you have figures from the time Tulip Mania happened? In fact, there are finite Tulips at any given point in time. Value is increased when goods become scares, either less units available, and/or demand increases. In reality: when demand is higher then supply, value goes up in a free market model.

Tulips are not easily portable

This argument has nothing to do with the comparison critics make: the comparison is about the enormous increase in value and a possible big bubble. And as a matter of fact we have to take the scenario of an enormous implosion of the Bitcoin value into consideration. Not saying it will not get out of an implosion though.

A tulip is not secured by millions of computers around the world

Sorry, again an argument that has nothing to do with the comparision between the Tulip Mania and Bitcoin. BTW, did you know that the majority of the mining power are owned be a handful of companies?

Bitcoin in a bubble?

It probably is, but the bubble may not burst. It really depends on the incoming fiat money from the risk takers from Wall Street and the likes. When they pull out their money, wait and see what happens with the Bitcoin price. But, they may not pull their money out of Bitcoin, they may even invest with Bitcoin in some Alts. Owww, I say invest; These are Traders, no Investors since in Cryptospace we hardly have investors other than all those people who invest with their time.