No rule covers a creature being within some distance of itself
While the Player's Handbook does say that, for instance, usually "references to allies include yourself" (308), there's no general mention, so far as I'm aware, of a creature always being within some distance of itself. However, an emanation like the 3rd-level Sor/Wiz spell magic circle against evil [abjur] (PH 249-50) typically includes oneself, and that's a lot like the effects of the feat Life Leech, but that's not definitive.
So the only real rules about such a thing are those that can be gleaned from the descriptions of other, similar feats. I searched my personal compilation of feats that covers what I think is all of the dnd-3e and dnd-3.5e feats, and a handful are similar to the feat Life Leech to reach a conclusion.
- The general feat Battlefield Inspiration (Miniatures Handbook 25) says that "you can inspire courage in… [e]ach ally within 30 feet of you (not
- The reserve feat Clutch of Earth (Complete Mage 40-1) has no restrictions upon whom the feat can be used—although it seems to assume opponents, it doesn't seem to prohibit the feat's possessor from using the feat on itself if it, for some reason, wanted to. Similarly, there appears to be no prohibition against a user employing the bardic music feat Sound of Silence (Complete Scoundrel 81) to deafen itself, although that feat, too, seems more aimed at enemies. Finally, a feline needn't use the feat Writhe (Web article “April Fools: Fabulous Cats!”) to convince itself to pet itself, but it's unclear—if the feline were somehow conflicted about the endeavor—if the feline could try to use the feat Writhe on itself, again the effect seemingly intended for foes. (How the feline employs—even on itself—an effect like the 3rd-level Sor/Wiz spell suggestion [ench] (PH 285)—which possesses the descriptor language-dependent—is a question for another time.)
- The general feat Commander (Dragon #323 27) and similar feats—about a half-dozen from the same article—often have a range of within 30 ft. of the feat's possessor but "don’t benefit the character who takes them; they only help his allies," of which, the creature is, of course, normally also one, but specific trumps general here.
- The domain feat Earth Devotion (Complete Champion 58) affects terrain within 30 ft. of the user, but doesn't prohibit the creature from affecting terrain in the user's own squares.
- The draconic feat Gold Dragon Lineage (Dragon Magic 19) notes specifically that it affects all allies including you within 30 ft. of you and says, "An affected ally need not remain within 30 feet of you to use this bonus."
- The general feat Music of Growth (Eberron Campaign Setting 57) says only that bonuses are granted by the feat's use to "every creature of the animal or plant type within 30 feet of you." Whether, for example, a tendriculos bard (I imagine it playing the xylophone) affects itself… is the same question raised by this question. Likewise the vile feat Slave to Evil (Elder Evils 14) when an overwhelming sign of the picked elder evil is present and the feat's possessor attempts to cast a divine spell. Also the possessor of the epic feat Souleater Incarnate (Epic Insights Web column “Epic Warlock Feats”) may—if its own feat affects itself—inadvertently devour its own soul if the GM so rules. And, finally, it's unclear if a vampire that—obviously consumed with self-loathing—possesses the feat Vampire Hunter (Libris Mortis 31) detects itself.
- The psionic feat Psionic Feedback (Dragon #316 80) that specifies that the feat's possessor does not "benefit from personally succeeding at a saving throw against a psionic power, whether manifested by you or someone else."
- The bardic music feat Warning Shout (Complete Scoundrel 82) specifies it doesn't work on the possessor despite it affecting an "ally… within 30 feet of [the feat's possessor that's] able to see or hear" him.
…And that's it for feats containing the text within 30 feet of you that don't also specify opponents. There're undoubtedly some that I missed, but I think the takeaway is that it seems like when a feat's benefit that works on creatures within 30 ft. doesn't want it working on the feat's possessor, the language often changes to opponents, making this feat—which deliberately specifies that it affects "[e]ach dying or stable creature within 30 feet of" the feat's possessor—a bit of an outlier.
However, as more feats specifically have language like not including you than have language like including you, the default seems to be including you, and that probably includes the feat Life Leech.
What happens if the the feat Life Leech affects the feat's possessor?
When there are no other dying creatures around and a creature that possesses the feat Life Leech has less than 0 hp, at the beginning of his turn, he loses 1 hp and gains for 10 min. 1 temporary hp. Then there's a 20% chance the creature stabilizes. If the creature doesn't stabilize, the creature loses 1 hp. Since temporary hp are lost first before real hp, the creature loses the just-gained temporary hp.
That's a net loss of 1 hp, and that's pretty much the same position the feat's possessor would've been in had he not had the feat Life Leech. So until the feat's possessor stabilizes at -9 hp—whereupon, without further intervention, next round on his turn he dies—the feat's possessor will notice little difference.