Do stimulants increase the IQ tests score for everyone?
IQ in ADHD sufferers indeed increases with the treatment of stimulants, such as methylphenidate (Meyes et al., 1994). This is thought to rely on the attention-narrowing effect of stimulants, i.e., they cause the 
 easily distracted ADHD child to focus better on the IQ task (Aman, 1996). 

It is an interesting question indeed whether such an effect also plays a role in healthy controls. Indeed it does, at least according to one study, where a control population was subjected to amphetamine. 


Amphetamine improves performance on spatial working memory, Stroop tasks and enhances language production (Barch &amp; Carter, 2005). The authors did, however, not measure performance on full blown IQ tests.


References
- Aman, J Development Phys Disabil (1996); 8(4): 347-65
- Barch &amp; Karter, Schizophrenia Res (2005); 77(1): 43-58
- Meyes et al., DMCN (1994); 36(12): 1099–107