With the renewed interest in functional programming languages, I've seen some similitudes between Smalltalk and FPL, namely closures ( BlockClosures in Smalltalk ) Yet, Smalltalk is not a FPL?
What would be needed to consider it as such?
Programming with the Object Oriented paradigm is creating a program by identifying and modeling the Problem Domain Entities as objects, and then make them collaborate between themselves to solve each problem instance. Programming with the Functional paradigm is modeling the problem as a mathematical problem, and creating a mathematical function (by composing other functions) that for each problem instance, computes the problem solution.
In my opinion, a functional programming language is a language that given a solution for a problem solved using the functional programming paradigm, the language can express that solution exactly as it was thought. If you need to "transform" some part of your solution so it fits in what the language can express, then it doesn't fully support the paradigm you used to think the solution.
Smalltalk can express in most cases all the solutions created using the Object Oriented Programming paradigm, but it cannot expresss primitively a lot of solutions created using the Functional Programming paradigm. That's why I don't consider it a FPL. Despite not primitively be able to express every solution that a FPL can, Smalltalk is extremelly extensible, and you can extend it to be able to express all the solutions a FPL can.