The mechanism is not very well understood, but it is thought that recombination occurs readily during replication by the RNA-dependent RNA polymerase that RNA viruses use for replication. In an infection in a single individual host organism, it would be expected to see recombinants between sister viruses, and in a co-infection of two or more closely related viruses, you would expect to find homologous recombinants present. Here is a ref for polio http://www.ncbi.nlm.nih.gov/pubmed/10329565
Thanks! I wonder if this viral recombination during replication has something in common with the recombination process during meiosis in eukaryotic cells.
If you want to read more about RNA recombination in plant viruses, I strongly recommend one of my publications that I made available for everyone on my profile. "RNA-RNA recombination in plant virus replication and evolution"
And probably you would like to check some reviews on mosaicism of dsDNA tailed phages (most studies are on T4). It seems that although different in DNA sequence, all tailed phages share a modular and functional structure. Their genes tend to cluster by function and are often exchanged in blocks.
Recombination (both intramolecular one or intermolecular one, reassortment) is widely used in several viral family (DNA virus and RNA virus).
In order to look at constraints, you might be interested in this paper :
Martin, D. P., E. van der Walt, D. Posada, and E. P. Rybicki. 2005. The evolutionary value of recombination is constrained by genome modularity. PLoS Genet. 1:475-9.