Actually there have been at least two articles written on this matter: Abraham Socher's "Shades of Frost" published in July 1, 2005 issue of TLS, and my own, "Frost and Shade, and Questions of Design," published in the Spring 2006 Nabokovian. Both these articles base the comparison between Shade and Frost on textual analysis of the poem "Pale Fire" and a number of Robert Frost's poems. In my article, I argue that references to Frost's poetry "provide yet another source for the poem [Pale Fire], a basis Shade can respond to as he opens up a dialogue with Frost. These allusions give us a better understanding of both "Pale Fire" and a number of Frost's poems, and create an important discourse on the driving forces of life and death, and the artistic design that governs them both." If you are interested, I can probably forward you the article, or you can find it in the Nabokovian.