pito:
This is with res dividers @ 2Mbaud, 20pF stray capacitance. You may use smaller resistors thus make edges even steeper..
Here is the analysis from Hermann Shutte, who first popularized the BSS138 MOSFet approach (at least to my knowledge). Those are pretty steep and well-defined edges, however!
All I recall (and it's been a while) is that the level-shifting issue would usually rear its ugly head in SD card applications and that the resistor networks that "classic" SD cards used to work with would stop working OK as SD card speeds advanced. Maybe Mr. Greiman can chime in? I note that the current ethernet shield now features the 74LVC1G125 buffer chip. Presumably, for a reason.
My guess is that it's not the Atmel IC that is the issue, it's likely the SD card that doesn't feature a nice 100MOhm input impedance like the ADC in the Atmel 328P. For me, since the primary reason I need level-shifting is logging to a SDHC card, using a TXB-series chip is second nature.