There is a nice little article in the Journal of Chemical Eduction [Negative pH Does Exist][1] pointing out examples of negative pHs in water, for example pH = -3.6 in mine water in California.  So -1.7 is not the lowest possible pH in water. Keep in mind that pH is defined in terms of hydrogen ion activity rather than concentration.  This is emphasized in [Paradoxes: Demonstrating That It Is Not True That pH ≡ -log\[H+\]][2].  At high acid concentation, hydrogen ion [activity][3] greatly deviates from hydrogen ion concentration.  For example, in 16 molar HCl, hydrogen ion activity is 678! 

And see [Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California][4] (no paywall), which describes pH values even below -4.

In [Hydronium ion activity in strongly acidic media. Remarkable agreement between independent estimates][5] Am. Chem. Soc. vol. 95, pp 3055–3057, 70% sulfuric acid is found to have a pH of -9.8 (negative 9.8)!


However, ignoring activity and the true definition of pH, considering that pure water has a concentration of 55M, if all the water molecules were instead hydronium ions, -log(55) = -1.7.  Then in articles like [Who Knows the Ka Values of Water and the Hydronium Ion?][6] this value is attributed to the pKa of hydronium, with the supposed justification that:
$K_a = \frac{[\ce{H3O+}][\ce{H2O}]}{[\ce{H3O+}]} = [\ce{H2O}]$

The article [New point of view on the meaning and on the values of
Ka(H3O+, H2O) and Kb(H2O, OH-) pairs in water][7] is a serious consideration of the true meaning of the pKa of hydronium, and points out that the -1.7 value for the pKa of hydronium has no justification. 

In conclusion, pH can be below -1.7 and the pK of hydronium isn't really -1.7.


  [1]: http://pubs.acs.org/doi/pdf/10.1021/ed083p1465
  [2]: http://pubs.acs.org/doi/pdf/10.1021/ed083p752
  [3]: http://en.wikipedia.org/wiki/Thermodynamic_activity
  [4]: http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1488&context=usgsstaffpub
  [5]: http://pubs.acs.org/doi/abs/10.1021/ja00790a076
  [6]: http://pubs.acs.org/doi/pdf/10.1021/ed063p473
  [7]: http://pubs.rsc.org/en/content/articlepdf/1998/AN/A705491B