They're called linux-headers-blahblah.
Code:
apt-cache search linux-headers
linux-headers-386 - Linux kernel headers on 386
linux-headers-686 - Linux kernel headers on PPro/Celeron/PII/PIII/PIV
linux-headers-686-smp - Linux kernel headers on PPro/Celeron/PII/PIII/PIV SMP
linux-headers-k7 - Linux kernel headers on AMD K7
linux-headers-k7-smp - Linux kernel headers on AMD K7 SMP
linux-headers-2.6.11-1 - Header files related to Linux kernel version 2.6.11
linux-headers-2.6.11-1-386 - Linux kernel headers 2.6.11 on 386
linux-headers-2.6.11-1-686 - Linux kernel headers 2.6.11 on PPro/Celeron/PII/PIII/PIV
linux-headers-2.6.11-1-686-smp - Linux kernel headers 2.6.11 on PPro/Celeron/PII/PIII/PIV SMP
linux-headers-2.6.11-1-k7 - Linux kernel headers 2.6.11 on AMD K7
linux-headers-2.6.11-1-k7-smp - Linux kernel headers 2.6.11 on AMD K7 SMP
linux-headers-2.6.10-5 - Header files related to Linux kernel version 2.6.10
linux-headers-2.6.10-5-386 - Linux kernel headers 2.6.10 on 386
linux-headers-2.6.10-5-686 - Linux kernel headers 2.6.10 on PPro/Celeron/PII/PIII/PIV
linux-headers-2.6.10-5-686-smp - Linux kernel headers 2.6.10 on PPro/Celeron/PII/PIII/PIV SMP
linux-headers-2.6.10-5-k7 - Linux kernel headers 2.6.10 on AMD K7
linux-headers-2.6.10-5-k7-smp - Linux kernel headers 2.6.10 on AMD K7 SMP
Now you can apt-get the appropriate one from the list.