I am able to lock Xserver on r300 and current Mesa in few easy steps:

1) start my application
2) move window application with mouse
3) quit the application

Application renders one frame - few textured rectangles, then waits for key.
If I disable rendering or if I won't move window with mouse - application closes correctly.

I was able to use gdb from second machine.
First, Xserver (I needed to use ctrl+c):

#0  0xb7f91410 in __kernel_vsyscall ()
#1  0xb7e30179 in ioctl () from /lib/libc.so.6
#2  0xb7befa55 in drmGetLock () from /usr/lib/libdrm.so.2
#3  0xb7f8287f in DRILock () from /usr/lib/xorg/modules/extensions//libdri.so
#4  0xb7f828fa in DRIDoWakeupHandler ()
   from /usr/lib/xorg/modules/extensions//libdri.so
#5  0xb7f8186e in DRIWakeupHandler ()
   from /usr/lib/xorg/modules/extensions//libdri.so
#6  0x0808a535 in WakeupHandler ()
#7  0x0819a114 in WaitForSomething ()
#8  0x08086621 in Dispatch ()
#9  0x0806e579 in main ()

Then top of my application backtrace:

#0  0xb7790ead in radeonGetLock (radeon=0x807a670, flags=0) at radeon_lock.c:62
#1  0xb77993ad in r300FlushCmdBuf (r300=0x807a670,
    caller=0xb78f54e6 "r300FreeGartAllocations") at r300_cmdbuf.c:117
#2  0xb779653f in r300DestroyContext (driContextPriv=0x80734e0)
    at r300_context.c:402
#3  0xb778eb9d in radeonDestroyContext (driContextPriv=0x0)
    at radeon_screen.c:933
#4  0xb778b2d7 in driDestroyContext (dpy=0x80518a8, scrn=0,
    contextPrivate=0x80734e0) at ../common/dri_util.c:751
#5  0xb7d30493 in DestroyContext () from /usr/lib/libGL.so.1
#6  0xb7dad312 in X11_GL_Shutdown () from /usr/lib/libSDL-1.2.so.0
#7  0xb7db1aa7 in X11_DestroyWindow () from /usr/lib/libSDL-1.2.so.0
#8  0xb7db1cc3 in X11_VideoQuit () from /usr/lib/libSDL-1.2.so.0
#9  0xb7da067e in SDL_VideoQuit () from /usr/lib/libSDL-1.2.so.0
#10 0xb7d74465 in SDL_QuitSubSystem () from /usr/lib/libSDL-1.2.so.0
#11 0xb7d744de in SDL_Quit () from /usr/lib/libSDL-1.2.so.0

From glxinfo:
OpenGL vendor string: DRI R300 Project
OpenGL renderer string: Mesa DRI R300 20060815 AGP 4x x86/MMX+/3DNow!+/SSE2 TCL
OpenGL version string: 1.3 Mesa 6.5.3

From .drirc:
<driconf>
    <device screen="0" driver="r300">
        <application name="Default">
            <option name="force_s3tc_enable" value="false" />
            <option name="texture_coord_units" value="8" />
            <option name="fthrottle_mode" value="2" />
            <option name="no_rast" value="false" />
            <option name="tcl_mode" value="3" />
            <option name="texture_depth" value="0" />
            <option name="disable_s3tc" value="false" />
            <option name="def_max_anisotropy" value="1.0" />
            <option name="command_buffer_size" value="8" />
            <option name="round_mode" value="0" />
            <option name="dither_mode" value="0" />
            <option name="disable_lowimpact_fallback" value="true" />
            <option name="texture_image_units" value="8" />
            <option name="no_neg_lod_bias" value="false" />
            <option name="color_reduction" value="1" />
            <option name="vblank_mode" value="1" />
        </application>
    </device>
</driconf>