Thanks for your fast answer!
I'm not sure, but the only thing which requires 5.2.1 now is PEAR::Net_IDNA2 package. The package is used when idn/intl module is not installed.
intl is not installed on my system. However it seems to work fine until now (maybe because I use the English interface? (I'm not familiar with NET_INDA2))
I will test some further and hope that the way I use roundcube is not affected by the php 5.2.1 dependency.
Thanks again!
Fred