Very odd. We have SAPGUI 7.30 pl8 hotfix 1 deployed pretty widely now, across a variety of machines running XP and Win7 (both 32-bit and 64-bit). Many of these have multiple monitors. In fact, I'm using it now with three monitors, and I frequently have multiple sessions open. We haven't seen anything like this.
I am using the same resolution on all three monitors, and all three are using the default resolution (they are LCD monitors). I do have users who don't use the default resolution (usually because they don't know about making font sizes larger, so instead they detune their monitors and ruin their eyes by staring at fuzzy fonts), and I have a small handful with differently-sized multiple monitors. I haven't had reports of this kind of problem from them.
What we have noticed is that some things do not work as expected when using the older themes (Enjoy), usually for someone who has upgraded from an older GUI and kept the same theme. It wasn't the problem you are describing, but still it did seem that the older themes were not fully compatible. That was on an earlier patch level, though, and since then I changed the upgrade script to reset everyone to the default (Signature) theme, so everyone's on the same level playing field. Since then there haven't been problems of this nature.