Below are some references. I previously posted these in December 2005 (URLs just below), but as I haven't kept up with this topic, it's possible that something relevant may have been published since 2005.
http://groups.google.com/group/alt.math.undergrad/msg/eb8efd19eebab8f0
http://mathforum.org/kb/message.jspa?messageID=4143759
[1] Eugen Dobrescu and Ioan Siclovan, Considerations on functions of two variables (Romanian), Analele Universitatii Timisoara Seria Stiinte Matematica-Fizica 3 (1965), 109-121. [MR 34 #5998; Zbl 166.31502]
[2] A. I. Fine and S. Kass, Indeterminate forms for multi-place functions, Annales Polonici Mathematici 18 (1966), 59-64. [MR 32 #7680; Zbl 137.03603]
[3] Ira Rosenholtz, A topological mean value theorem for the plane, American Mathematical Monthly 98 (1991), 149-154. [MR 91m:26014; Zbl 741.26003]
[4] Tadeusz Wazewski, Une généralisation des théoremes sur les accroissements finis au cas des espaces abstraits. Applications, Bull. Int. Acad. Polon. Sci. Lett., Cl. Sci. Math. Natur., Ser. A 1949, 183-185. [MR 12,508a; Zbl 41.23301]
[5] Tadeusz Wazewski, Une généralisation des théorèmes sur les accroissements finis au cas des espaces de Banach et application à la généralisation du théorème de l'Hôpital, Ann. Soc. Polon. de Math. 24 (1951), 132-147. [MR 15,717g; Zbl 52.11302]
[6] Tadeusz Wazewski, Une modification due théorème de l'Hôpital liée au problème du prolongement des intégrales des équations différentielles, Annales Polonici Mathematici 1 (1954), 1-12. [MR 16,118e; Zbl 56.11402]
[7] William H. Young, On indeterminate forms, Proceedings of the London Mathematical Society (2) 8 (1910), 40-76. [JFM 40.0334.01]
from p. 71 of Young's paper: "We now pass to one or two generalisations to more than one variable. It has commonly, but erroneously, been supposed, that such generalisations did not exist."