So, during a conversation at PyCon UK, Keith White noticed that he owned the same model (HP nx6325) laptop as me. He also informed me that his processor was 64-bit, and asserted that mine probably was as well.

I was sceptical. It's a Mobile Sempron - last I checked, all Semprons were 32-bit. Of course, I don't keep up with hardware at all. Checking /proc/cpuinfo:

tim@regulus:~$ cat /proc/cpuinfo <br />
processor       : 0<br />
vendor_id       : AuthenticAMD<br />
cpu family      : 15<br />
model           : 76<br />
model name      : Mobile AMD Sempron(tm) Processor 3500+<br />
stepping        : 2<br />
cpu MHz         : 800.000<br />
cache size      : 512 KB<br />
fdiv_bug        : no<br />
hlt_bug         : no<br />
f00f_bug        : no<br />
coma_bug        : no<br />
fpu             : yes<br />
fpu_exception   : yes<br />
cpuid level     : 1<br />
wp              : yes<br />
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow up pni cx16 lahf_lm extapic cr8_legacy ts fid vid ttp tm stc<br />
bogomips        : 1597.70<br />
clflush size    : 64

The 'lm' flag means 64-bit. So intuitive and obvious... so I guess that's one more thing to try when I get around to reinstalling my laptop.