On this computer, I have:
-Windows XP, Media Center Edition
-Muse Score Prerelease revision 1944

In this revision, it is possible to create cross staff arpeggios. However, if I create an arpeggio from a chord on the top staff, then drag it down to the bottom staff, I can still only select it from the top staff. I've attached a pdf of the problem. To reproduce:

1) Create a chord on both staves of a piano
2) Create an arpeggio on the top chord
3) Drag it down to the bottom chord
4) If you click on the part of the arpeggio in the bottom of the staff, it will not select, however, if you click on the arpeggio at the top part of the staff, it will select.

File attachment: arpeggio.pdf

Reported by issac: http://musescore.org/en/node/2256