PA-RISC Linux supports all HP PA-RISC workstations. Not everything is perfectly functional - but nearly all of it works. The unlettered 700-series workstations (eg 712/715/735) and the B/C/J class machines (e.g. B132L, Tadpole B160L laptop, C180L, C200, B1000, B2000, C3000, J5000, C8000) systems are supported. Fast-Wide Differential SCSI does not work on 735 or 755 (but does on other machiines).
Most standard HP graphic cards work (Visualize-EG, Artist, HCRX, ...) in 8 and 24bpp modes. Support for the Vis-FX series is still missing and will probably not be supported.
PA-RISC VME boards are based on workstation designs. PA-RISC Linux does boot on a 743.
The earliest PA-RISC servers have proprietary HP devices attached to proprietary HP bus architectures. Documentation on these busses and devices will not become available soon since no one in HP is finding, "scrubbing", and releasing the docs. Machines in this category are the F, G, H, I class (aka Nova) and T500 series (Emerald) machines as well as some earlier, unlettered servers.
The T600 is unlikely to ever be supported. Though T600 uses some of the same GSC devices (eg card-mode Dino 100BT, Zalon FWD SCSI) as K-class, T600 has the Java I/O MMU, instead of U2, which is NOT I/O coherent. Linux PCI drivers require I/O coherency.
Support for the E class is being pursued. Similar to K class in some ways, it requires the Mux driver for console and does not yet have SCSI or Centronics support. Network support is the same as 712. GSC add-in cards should work, HP-PB cards will not yet.
The D, K and R class machines are at least partially supported. These machines are architecturally similar to the older B, C and J class workstations and share most of the I/O devices, busses (GSC) and processors. The console on K class and the optional console card on D/R class are now supported. Most of the other HP-PB (aka NIO) devices aren't supported due to lack of documentation -- the same documentation which makes the Nova/Emerald machines unsupportable.
The A180, L1000/2000/3000, N4000, and A400/A500 servers are supported.
Support for Superdome might someday follow but requires some additional VM work. Superdomes have a cellboard based architecture. There was never enough documentation released that the according memory management could be programmed in Linux. So probably Superdomes may not work. Maybe on an physical partition level, there might be a chance, since the single cellboard is pretty similar to the L-Class. One idea is to try to get the partition management running and install one partition HP-UX and then try to work with linux on an additional partition.
People ask about V-class. V-class has a very different architecture from other HP machines. See the Server Architecture Reference for details. One major known problem with supporting V-class is the complexity of its PCI bus adapter (EPIC and SAGA). Support for the V-class isn't going to happen until some really masochistic person who has access to a V-class is, uh, challenged by this.
A list of currently supported hardware is maintained by the ESIEE Team, you should take a look here for any precise material.