Querying STRINGdb for a set of given gene names, and plotting the network works fine:

require(STRINGdb)
myDF <- data.frame(gene = c('AAAS', 'AACS', 'AAMP', 'AASS', 'ABCB7', 'ABCD4'))
mapped <- string_db$map(myDF, 'gene', removeUnmappedRows = TRUE)
string_db$plot_network(mapped$STRING_id)

Q: How to export this network to XML (for import into cytoscape)? I tried searching for such functionality, but couldn't find it:

STRINGdb$help("export")
ls('package:STRINGdb')
?`STRINGdb-class`
?`STRINGdb

Thankyou for using STRING.

Good question. We don't have such functionality at the moment (we take your suggestion into our wish list).

However a STRING network in R, is just an iGraph object (http://igraph.org).

With the function "get_subnetwork(string_ids)" you can obtain an iGraph object of the subnetwork generated with the STRING identifiers that you provide to the function.

Then you could in principle convert such iGraph object to a format that can be accepted by Cytoscape, or use plugins like (http://rcytoscape.systemsbiology.net).