Hmm, could you take a look in System/Library/Extensions/ATIRNRDV.kext? Open it with right-click/Show Package Contents and take a look in Contents/Plugins. What do you have in there? I have ATY_Megalodon, ATY_Prionace and ATY_PrinaceB.

You can download a package with modified drivers here. These are the drivers from my 2600XT, but Natit.kext is modified, it uses the Megalodon framebuffer for the 2900XT instead of the Franklin framebuffer. Maybe this works. Copy the files into the Extensions folder, repair permissions and reboot with -f option or in safe mode -x.

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2 Opteron systems: OSx86 10.5.8, Andy's 9.8.0 kernel, Asus A8N-SLI Premium, Opteron 185 o'clocked @ 2 x 2,95 GHz (2nd system 2.6 GHz), ATI Radeon HD2600XT 256MB Dual-Monitor 2x HP L2035, 4 GB RAM, Griffin FireWave as main audio device, Marvell + nForce LAN, Asus U3S6 USB3/SATA6 card, 5,5 TB harddisk, Firewire 800 card, Apple Remote + eHome IR receiver, 2x Wacom serial graphics tablet, Canon Pixma iP4700, Logitech Internet Navigator wireless keyboard/mouse combination.

My Audio stuff: M-Audio Transit USB (default audio), M-Audio ProFire 610, M-Audio ProFire Lightbridge (34 channels) using Creamware A16 ADAT converter • MIDI: M-Audio Midiman 4x MIDI interface • Behringer Audio Mixers: Xenyx 1002, Xenyx 1002FX, Xenyx 1202FX, Eurorack UB1002FX, Eurorack MX1804FX, Eurorack MX262A • FX devices: Lexicon MPX100 DSP, Behringer DSP-1000 Virtualizer, Behringer MiniFEX 800 DSP, Behringer Multicom Pro MDX4400 compressor • RETRO: MSSIAH midi/sequencer/synthesizer cardridge for the C64 (Dual-SID), Steinberg M.S.I. MIDI Interface for C64