I discovered recently why most of the ambushes that I've set up during 3rd Edition have failed spectacularly, falling foul of such lethal PC weapons as... Spot checks, and that mother of all annoyances, Scent.
While I can't do anything about the latter (although you could make it harder for an ambush to be sniffed out using magical wind or other extraordinary effects), the answer to the former is spectacularly obvious and something that I forget about on a regular basis: Take 20.
There's nothing in the SRD's Hide entry that says you can't Take 20, but there are a few complications, most notably that the Hide skill is in effect an opposed roll (versus your opponent's Spot check)... therefore how would one be able to repeatedly retry when the outcome of any one try is unresolved until your opponent wanders into shot? One possible answer is that you can only Take 20 on a Hide check if you have someone available to help... which then has other benefits, namely the use of Aid Another to get a further +2 bonus (or bonuses, from multiple allies) to help you with your effort.
However all of those factors are ignoring the obvious, in-game logic which is that given enough time and opportunities for cover or concealment, a skilled exponent can scout out a decent place to hide, secrete himself within it, cover his tracks, and use whatever detritus is available at hand to cover himself up. If he couldn't, then a villain who dove behind a bush with the PC's a few seconds behind him would have exactly as much chance of concealing himself as a ninja in active camouflage who took a day to dig a foxhole and cover the entrance with dried twigs.
Conclusion: characters, NPC's, and monsters can Take 20 on a Hide check as long as they have 2 minutes in which to do so. This makes ambushes suddenly very viable, as creatures with even average Hide checks (say, +8 to +10) are now setting Spot DC's in the high 20's/low 30's; meanwhile villains with exceptional Hide abilities will be practically impossible for all but the most experienced adventurers to spot, setting DC's in the 40's or 50's quite easily.
And that all sounds just fine to me.