A kernel from a month ago worked fine, but when I tell my modern kernel
ifconfig media 10baseT, the link ok light on the hub goes out
immediately. The only way to get the interface into a working state is
to reboot the box entirely. AFAICT this only affects my ne2000, the
3c509 I also have in the same box seems to work fine even after ifconfig
media 10baseT. If I do ifconfig ne0 without the media argument, the
interface works ok.

I can't tell the exact time this happened, since I was without a new
kernel for a bit over a month. The last known working kernel was a 1.4T.
The related(?) dmesg change looks like this:
! ne0: 10base2, 10baseT, 10baseT-FDX, auto, default auto
! ne0: 10base2, 10baseT, 10baseT-FDX, auto, default [0x00 0x30] auto

Before I send-pr this I'd like to know if anyone else has been
experiencing it.

Here are the relevant parts of dmesg:
NetBSD 1.4X (BABYLON5) #98: Mon Mar 27 20:27:21 EEST 2000
    pooka@babylon5:/usr/src/sys/arch/i386/compile/BABYLON5
ne0 at pci0 dev 10 function 0: RealTek 8029 Ethernet
ne0: 10base2, 10baseT, 10baseT-FDX, auto, default [0x00 0x30] auto
ne0: supplying EUI64: 00:80:ad:ff:fe:38:eb:14
ne0: Ethernet address 00:80:ad:38:eb:14
ne0: interrupting at irq 12
ep0 at isa0 port 0x300-0x30f irq 10: 3Com 3C509 Ethernet
ep0: address 00:60:8c:cf:1f:f3, 8KB byte-wide FIFO, 5:3 Rx:Tx split
ep0: 10baseT, 10base5 (default 10baseT)

-- 
Antti Kantee <pooka@iki.fi>