We report observations of three SDSS $z>6$ QSOs at 250 GHz (1.2 mm) using the 117-channel Max-Planck Millimeter Bolometer (MAMBO-2) array at the IRAM 30-meter telescope. J1148+5251 ($z=6.42$) and J1048+4637 ($z=6.23$) were detected with 250 GHz flux densities of $\rm 5.0 \pm 0.6 \,~mJy$ and $\rm 3.0 \pm 0.4 \,~mJy$, respectively. J1630+4012 ($z=6.05$) was not detected with a $3~\sigma$ upper limit of 1.8 mJy. Upper flux density limits from VLA observations at 43 GHz for J1148+5251 and J1048+4637 imply steeply rising spectra, indicative of thermal infrared emission from warm dust. The far-infrared luminosities are estimated to be ≈$10^{13} \, L_\odot$, and the dust masses ≈$ 10^8 \, M_\odot$, assuming Galactic dust properties. The presence of large amounts of dust in the highest redshift QSOs indicates that dust formation must be rapid during the early evolution of QSO host galaxies. Dust absorption may hinder the escape of ionizing photons which reionize the intergalactic medium at this early epoch.