There is no longer a "ppp" module in 2.4.0. Hence pppd should try to modprobe ppp_generic (if ppp fails) in case the kernel is >= 2.2.4.
*** Bug 22840 has been marked as a duplicate of this bug. ***
This should be fixed by an internal ppp->ppp_async (which should depend on ppp_generic) alias in new modutils packages. Please reopen this bug if you find that this is not the case with modutils-2.4.0-1 and later.