For the Dell R610s we're upgrading specifically, it is supported as indicated by the R610 technical guide:
With the introduction of the Intel
processor 5600 series, low voltage (LV) DIMMs have been added for selected memory configurations for the PowerEdge R610. Only this processor series supports operating DIMMs at the lower voltage (1.35V, also referred to as DDR3L). The Intel Xeon 5500 processor series does not support low voltage operation. However, due to the backwards compatible nature of low voltage DIMMs, they can be operated at 1.5V. Therefore, DDR3L DIMMs can
be used in systems with either processor series, and the platform will automatically choose the appropriate operating voltage based on the processor populated. DDR3L DIMMs will be qualified and available for use with Intel Xeon 5500 processor series mid-year 2011.
Also, we've confirmed this by just installing such LV DIMMs in our web tier along-side non-LV DIMMs and all is well.
This may vary by server, but the way this is worded in several technical guides, it appears to be universally true that LV DIMMs will work at a normal 1.5V level.