Gmail's chat now comes with a send SMS feature that allows you to send sms to smart and sun networks.  You start off with 50 sms credits. If you send out an sms, your credit goes down 1. If someone replies, you get 5 credits back. Not sure yet how much a reply will cost the sender but it is not bad. You can reload by replying to your own message i guess. Cheaper than the 1 peso it costs to reload for some networks.

 For more details please check out http://www.google.com.ph/intl/en/mail/help/sms.html

 Update: Replies will cost only 1 peso. With sun cellular, you need to use a regular load to reply and not your unlimited text plan.  Your credit maxes out at 50. If you get a reply with 49 credits left, you wont get 5 credits back but just the 1 to fill up the 50 max.