Had this laying around for quite a while -- really useful and kind of dumb it's not built in...

gistfile1.m

// stringByAddingPercentEscapesUsingEncoding is pretty conservative about escaping characters, which is fine most of the time. This method however escapes *all special characters*, allowing any arbitrary string to be used when building any part of a URL.
// (c) 2011 Springs & Struts. Released under MIT-style license. See: http://www.opensource.org/licenses/mit-license.php

@interface NSString (SSURLEscaping)
- (NSString *)ss_stringByEscapingAllURLCharactersForRealWithEncoding:(NSStringEncoding)encoding;
@end

@implementation NSString (SSURLEscaping)

- (NSString *)ss_stringByEscapingAllURLCharactersForRealWithEncoding:(NSStringEncoding)encoding
{
    return [(id)CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (CFStringRef)self, NULL, CFSTR(";/?:@&=+$,"), CFStringConvertNSStringEncodingToEncoding(encoding)) autorelease];
}

@end