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.
Swap support finally allows building Haiku in Haiku on a box with less than about 800 MB RAM, as long as as the swap file is large enough. I tested this on a Core 2 Duo 2.2 GHz with 256 MB RAM (artificially limited) and a 1.5 GB swap file. Building a standard Haiku image with two jam jobs (jam -j2) took about 34 minutes. This isn't particularly fast, but Haiku is not well optimized yet.
Haiku's swap implementation was heavily inspired by that of FreeBSD. At the moment it is not as sophisticated, but Zhao intends to borrow more of FreeBSD's optimizations.