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The strength of the He I 2.06 $\mu$m emission line relative to hydrogen recombination features has been previously proposed as a diagnostic of effective temperature $T_{eff}$ for stars powering HII regions. Such an indicator would be of particular interest for constraining the high-end initial mass function in starbursts. This talk will present new calculations bearing on the sensitivity of the 2.06 $\mu$m feature to nebular conditions. The results illustrate a number of complications for interpreting the He I/Br$\gamma$ ratio as a constraint on $T_{eff}$. The ratio is sensitive to the relative volumes of He$^+$ and H$^+$, as well as the relative ionization fractions of He and H within the He$^+$ zone, both of which respond to variation in $T_{eff}$. Predicted values of He I/Br$\gamma$ are also sensitive to nebular geometry, dust content, density, and helium abundance. The present examples suggest that caution is required in intepreting measured ratios as indices of ionizing continuum shape.
