Manufacturing with a mixed metal system is, unfortunately, becoming more common as high volume manufacturers move to Pb-free. This in turn causes the package manufactures to produce predominately Pb-free bumped packages. When these are used in a Sn/Pb process there can be a significant impact on reliability.
A worst case scenario is where the SAC bump is not reflowed or only partially reflowed. This typically occurs when a "standard" Sn/Pb reflow process is used, the incomplete mixing will have a significant impact on reliability Hillman, et al. (CMAP, Toronto, 2005) showed the disastrous consequences of not completely reflowing the solder joint and having a Pb-rich region in the solder joint.
More recently Pan, et al. (IPC/JEDEC, San Jose, 2005) summarized some of the previous work, including reliability studies done by Solectron where they showed an impact in reliability, but in some cases may be acceptable, as long as the correct temperature is achieved in order to get complete mixing. This paper also provides the methods to calculate the reflow temperature needed in order to get complete mixing (based on the bump size, alloy, and paste deposit). Of course it is going to be higher than a standard tin-lead reflow temperature. Therefore you need to insure that you Sn/Pb flux system is capable of withstanding the higher temperatures without suffering flux exhaustion.
There are a few vendors today who offer "transition" solder pastes, that are Sn/Pb alloy based, with a flux medium that can survive higher temperatures. Profile the PCB (with all the components populated) to be sure the BGA solder joints achieve the desired temperature, then cross section the solder joints to make sure the solder joint looks homogenous. Depending on your end customer you may need to provide reliability data.