Is there an issue that the GPL code would 'virally' infect the instance of the boost code in the combined source such that the boost code being redistributed would need to have the same (ie additional) restrictions as the GPL.
Theoretically, perhaps, in Practice no. Consider this: You want to create a Product (lets call it P) it consists of three parts: Part Y - your own Code (i.e. you hold the copyright to this Part.) Part B - the Boost Library. (Has the Boost License) Part G - Some Code which is GPL Licensed. So the question (of the OP) is, what license does P stand under? Boost says: We don't care, use whatever you like. GPL says use GPL. So P has to be licensed under the GPL. So is the license of part B "infected" by G (for this Instance)? (Theoretically) Yes. (and ONLY for this Instance, the original is never modified)
Someone is free to also get Boost code directly from Boost and not have these restrictions, but does the Boost code allow a user to place further restrictions on the derived work?
Is this a problem? No. The Boost License allows this.
So now everyone who uses this Instance of Boost would have to use the GPL? Theoretically. In practice, how can you tell which instance of Boost someone uses? So no, everybody is still free to use Boost however he wants to.
Paul (not a lawyer, and always confused by licenses)
Fabio, also not a lawyer.