Dojo can URL encode the arguments list for you, like so:
function getRSS(id,url,limit){
var obj = document.getElementById(id);
dojo.io.bind({
url: "http://localhost/rss/getRSS.php",
content: {rssurl: url, rsslimit: limit},
load: function(type, data, evt){
RSSLoaded(obj,data); // error handling needed
},
mimetype: "text/plain"
});
}