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.
This is a companion discussion topic for the original entry at https://www.haiku-os.org/news/2008-08-29_haiku_grows_swap_support/