> Could you please try booting an INSTALL kernel and confirm
> whether fxp0 functions normally?

Good idea. Did so: did not work. So it's not a netbooting problem,
but the fxp driver does not support my card currently.

> Do you have any idea when this last worked in -current?

No, sorry, though I seem to recall the latest MP test kernel still worked.

When booting the install kernel I got hold of the kernel output (below)
and checked the autonegotiation stuff: 

fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
	address: 00:a0:c9:25:13:84
	media: Ethernet autoselect (100baseTX full-duplex)
	status: active
	inet 212.8.203.83 netmask 0xfffffff8 broadcast 212.8.203.87
	inet6 fe80:1::2a0:c9ff:fe25:1384 prefixlen 64

100baseTX/full duplex is correct and the same is indicated by the switch.

Dmesg is:

    martin@rumolt.teuto.de:/usr/src/sys/arch/i386/compile/INSTALL
cpu0: family 6 model 5 step 2
cpu0: Intel Pentium II (Deschutes) (686-class)
total memory = 255 MB
avail memory = 232 MB
using 3296 buffers containing 13184 KB of memory
BIOS32 rev. 0 found at 0xfb2e0
mainbus0 (root)
pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled
pchb0 at pci0 dev 0 function 0
pchb0: vendor 0x8086 product 0x7190 (rev. 0x03)
ppb0 at pci0 dev 1 function 0: vendor 0x8086 product 0x7191 (rev. 0x03)
pci1 at ppb0 bus 1
pci1: i/o space, memory space enabled
vendor 0x121a product 0x0003 (VGA display, revision 0x03) at pci1 dev 0 function 0 not configured
pcib0 at pci0 dev 7 function 0
pcib0: vendor 0x8086 product 0x7110 (rev. 0x02)
pciide0 at pci0 dev 7 function 1: Intel 82371AB IDE controller (PIIX4) (rev. 0x01)
pciide0: bus-master DMA support present
pciide0: primary channel wired to compatibility mode
wd0 at pciide0 channel 0 drive 0: <IBM-DTTA-351010>
wd0: drive supports 16-sector pio transfers, lba addressing
wd0: 9671 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 19807200 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2
wd1 at pciide0 channel 0 drive 1: <WDC WD205AA>
wd1: drive supports 16-sector pio transfers, lba addressing
wd1: 19569 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 40079088 sectors
wd1: 32-bit data port
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 4
pciide0: primary channel interrupting at irq 14
wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 2 (using DMA data transfers)
wd1(pciide0:0:1): using PIO mode 4, Ultra-DMA mode 2 (using DMA data transfers)
pciide0: secondary channel wired to compatibility mode
atapibus0 at pciide0 channel 1
cd0 at atapibus0 drive 1: <CRD-8322B, 1998/07/17, 1.02> type 5 cdrom removable
cd0: 32-bit data port
cd0: drive supports PIO mode 4, DMA mode 2
pciide0: secondary channel interrupting at irq 15
cd0(pciide0:1:1): using PIO mode 4, DMA mode 2 (using DMA data transfers)
vendor 0x8086 product 0x7112 (USB serial bus, revision 0x01) at pci0 dev 7 function 2 not configured
vendor 0x8086 product 0x7113 (miscellaneous bridge, revision 0x02) at pci0 dev 7 function 3 not configured
vendor 0x1102 product 0x0002 (audio multimedia, revision 0x05) at pci0 dev 9 function 0 not configured
vendor 0x1102 product 0x7002 (miscellaneous input, revision 0x05) at pci0 dev 9 function 1 not configured
fxp0 at pci0 dev 11 function 0: Intel i82557 Ethernet, rev 1
fxp0: interrupting at irq 12
fxp0: Ethernet address 00:a0:c9:25:13:84, 10/100 Mb/s
nsphy0 at fxp0 phy 1: DP83840 10/100 media interface, rev. 0
nsphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
isa0 at pcib0
com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo
isapnp0 at isa0 port 0x279: ISA Plug 'n Play device support
npx0 at isa0 port 0xf0-0xff: using exception 16
pc0 at isa0 port 0x60-0x6f irq 1: color
pc0: console
fdc0 at isa0 port 0x3f0-0x3f7 irq 6 drq 2
fd0 at fdc0 drive 0: 1.44MB, 80 cyl, 2 head, 18 sec
isapnp0: no ISA Plug 'n Play devices found
biomask efe5 netmask ffe5 ttymask ffe7
md0: internal 2048K image area
boot device: <unknown>
root on md0a dumps on md0b
root file system type: ffs