Using Mesa git from 06/21/2010 causes an X server crash when a client trying to use  AIGLX exits.  This is using Ubuntu 10.04, with all the latest updates from Ubuntu.

Even with xserver-xorg-core-dbg installed, the backtrace only gives me:

Backtrace:
0: /usr/bin/X (xorg_backtrace+0x28) [0x4a3238]
1: /usr/bin/X (0x400000+0x655ad) [0x4655ad]
2: /lib/libpthread.so.0 (0x7f3e91f96000+0xf8f0) [0x7f3e91fa58f0]
3: /usr/lib/dri/r300_dri.so (0x7f3e8e2f5000+0x2ed09) [0x7f3e8e323d09]
4: /usr/lib/dri/r300_dri.so (0x7f3e8e2f5000+0x121d2c) [0x7f3e8e416d2c]
5: /usr/lib/dri/r300_dri.so (0x7f3e8e2f5000+0x1fccd) [0x7f3e8e314ccd]
6: /usr/lib/dri/r300_dri.so (0x7f3e8e2f5000+0x1b200) [0x7f3e8e310200]
7: /usr/lib/xorg/modules/extensions/libglx.so (0x7f3e8f91a000+0x40f99) [0x7f3e8f95af99]
8: /usr/lib/xorg/modules/extensions/libglx.so (0x7f3e8f91a000+0x3668a) [0x7f3e8f95068a]
9: /usr/lib/xorg/modules/extensions/libglx.so (0x7f3e8f91a000+0x366d3) [0x7f3e8f9506d3]
10: /usr/bin/X (FreeClientResources+0xd3) [0x44d733]
11: /usr/bin/X (CloseDownClient+0x60) [0x42c3e0]
12: /usr/bin/X (0x400000+0x30ad0) [0x430ad0]
13: /usr/bin/X (0x400000+0x261aa) [0x4261aa]
14: /lib/libc.so.6 (__libc_start_main+0xfd) [0x7f3e90c8ec4d]
15: /usr/bin/X (0x400000+0x25d59) [0x425d59]
Segmentation fault at address 0xb00000001

It worked on Thursday the 17th, and a git bisect reveals:

 git bisect good
d82f6253331abf09ca714b844b1a9179ed8050b3 is the first bad commit
commit d82f6253331abf09ca714b844b1a9179ed8050b3
Author: Marek Olšák <maraeo@gmail.com>
Date:   Mon Jun 21 00:42:06 2010 +0200

    r300g: fix the KIL opcode for r3xx-r4xx (v4)

:040000 040000 ecc1d75ccfc8a09fc24c37ecb3dcb2ed0f449dbf 1456ca25ec93971da8e636b8c674d1202bcf7540 M      src