It seems that Google Summer of Code has payed up for Haiku which now has swap support.

From the announcement:

Thanks to Google Summer of Code student Zhao Shuai successfully finishing his project Haiku does now feature support for swapping. As of revision 27233 it is enabled by default, using a swap file twice the size of the accessible RAM. The swap file size can be changed (or swap support disabled) via the VirtualMemory preferences.