I'm willing to bet that it has no limitation on use but is triggered by certain abilities that are limited. I'm also willing to bet that it doesn't increase by level and that the abilities that trigger healing surges do.
For example, a 10th-level rogue with 14 Con has a healing surge modifier of 8. A cleric comes along and heals him using Cure Serious Wounds which triggers the rogues healing surge with a x4 multiplier, giving the rogue 32 health back. He then casts CSW again and again heals the rogue of 32 hit points. The Warlord shouts out some epithet that inspires everyone to greatness (thus using a 1/encounter ability), that gives everyone a healing surge with a x3 multiplier, thus healing the rogue of 24 hit points.
There could also be bonuses to other peoples or your own healing surges via other abilities. So for instance, Cure Serious Wounds might remain a x4 multiplier but add +1 per 2 levels of the caster to the person's healing surge. So in the case of the above example, the rogue would receive 52 hit points instead of 36. Items could also give bonuses to healing surges, etc. This would scale things well for level making lower level spells always useful.
Obviously I'm just guessing amounts but it seems to me the simplest explanation which fits with all we've been told so far. It is also a very elegant and easy mechanic to work with.