It is my understanding that a calculator is provided in the 70-291 test environment.
But there is some debate over whether the provided calculator will do dec -> bin conversions in scientific mode.
According to this post (from 2005), it might depend on your test provider:
http://groups.google.com/group/microsoft.public.cert.exam.mcsa/msg/07186109ddfea81e
They said (again, in 2005)
Pearson Vue: yes
Prometric: no
So what's the real answer these days?
When you go to take the 70-291 exam, try the calculator and report back right here whether it could do scientific mode or not!
Give:
Test (if not 70-291)
DateTest Vendor (Prometric or Vue)
Calc mode: (basic-only, or sci-capable)