Ionic bonds- are electrons transferred?
This is not a simple question at all, in fact, it is an interesting insight in chemical bonding.  You make a valid assertion with your question:


  are ionic bonds just extremely polar covalent bonds?


This answer is a simplistic way of looking at it.  

According to ChemGuide page Electronegativity, there is no real clear-cut 'boundary' between covalent and ionic bonding. The website describes that there is a 'spectrum of bonding' based on the electronegativity (chart shown below) of the constituent elements.



Image source: UC Davis ChemWiki

So, the greater the electronegativity, the greater the polarity or "pulling power" on the bonding electrons towards elements with a higher electrongativity.

In the following diagrams (from the ChemGuide site linked before), A and B represent arbitrary elements, and the green and red dots represent bonding electrons. Starting from a situation where the polarity is zero as the electronegativity of the constituent elements are the same (e.g. diatomic molecules $\ce{O2}$, $\ce{Cl2}$ etc), as in diagram I. 

As the polarity increases, the bonding electrons move towards the more electronegative member, causing the bond to become increasingly polar (II), then eventually to the point where the difference is enough (III) for:


  To all intents and purposes, A has lost control of its electron, and B has complete control over both electrons. Ions have been formed.


I bolded the 'to all intents and purposes' to emphasise that this is a simplistic generalisation

(I)


(II)


(III)


As the lowest electronegativity is not zero (it is Francium at 0.7), there will always be an influence on the bonding electrons. So, the reality is that A does not completely lose its influence on the bonding electrons.

For example, if we were to look at a compound combining the most and least* electrongative elements (as appear in the diagram above): francium fluoride $\ce{FrF}$, even though fluorine has a far stronger electronegativity than francium, the electronegativity of the latter is not zero, so, the influence of the francium is very small, but not zero - indicating that purely ionic bonds don't exist.


As noted in the comments, cesium is likely to be less electronegative than francium, however the point made above still stands.


This is nicely summarised by an article from the University of Malta:


  Pure ionic bonding is not known to exist. All ionic compounds have a degree of
  covalent bonding. The larger the difference in electronegativity between two
  atoms, the more ionic the bond.