To expand on Rene's comment, there's no way around RoHS compliance
. Molex, for instance, has obsoleted all it's high density parts (to my knowledge) in favour of new RoHS compliant parts. They are probably doing this across the board, as they don't want to keep different stocks on hand with the logistics nightmare involved.
As to high speed problems:
Different materials will undoubtedly cause impedance vectors to change. In addition, the loss tangents across materials (very important in extremely high frequency designs) will vary. Whether they will be better or worse is unknown, but after all the research into existing materials, my take is they will be worse.
IC packaging will have to change. Most existing plastic packages have absolute maximum temperature ratings of about 220C, but the RoHS reflow profile is well above this (about 260C). See this Xilinx app note:
Changing the materials to make a device compatible with those reflow characteristics is bound to have an effect somewhere, although I don't know the specifics for Xilinx.
In addition to that, the solder joint of a tiny part (such as a coupling cap) has parasitics that are (somewhat) material dependent. Change the materials, change the parasitics. This too is an issue for high speed designs.
Note - when I mention extremely high frequencies, I am referring to bit times (on the wire pair) of 1nSec or less (PCI express, InfiniBand and Fibre channel come to mind).
So will it affect us in high speed designs? Undoubtedly.
The issue is we probably won't know the effects until there are large numbers of boards already made.