I find boundary mounting a useful technique to control echoy ambiences and add some presence that would be laking with omnis in other cofigs, and though not usually applicable for
typical stealth recording, mics mounted on a surface instead of a stand are very unobtrusive and often invisible.
It's not the usual way I use them, but I've used dpa 4060's in boundary mounted situations for concert recording with great results. I sometimes use the rubber DPA boundary mount adapters made for this purpose, but other times I just place or tape the mics to the surface (the mounts add a bit of additional boost around 10khz because of the tiny sound opening slot I suspect).
One situation had a concrete wall directly behind the soundboard and taping section at the rear of a shallow room. I taped the mics in the boundary mounts to the wall separated by about 3'. The elimination of the bounce off the rear wall plus the added clarity and slightly reduced ambient ratio due to the boundary mounting worked much better than anywhere else in that case.
They are excellent mics for recording in many situation - not stealth, though.
One scenario I use regularly, similar to what Javier mentioned, works extremely well for a jazz trio on an outside patio. We sit at a cafe table directly in front of the band and patrons pass between us and the band to enter the cafe. I treat it as a non-open taping situation even though it could be, for several reasons -elimination of setup, not wanting to draw attention to recording for both the patrons and band's sake (the band knows), and believe it or not I can get superior mic placement this way. I place the mics just above the floor in my shoes which gets closer to the band, provides nice boundary effect clarity for the the drums and especially the guitar and organ amps which are also on the floor, and 'looks though' the least shadowing portion of the people passing between us and the band - their ankles.
Granted that application doesn't apply to many situations but works perfectly in this case.