[2014 January] UPDATE : new Openlayers 3 beta 1 works quite well on iPad as well as Laeflet 0.7, check post here: http://blog.sumbera.com/2014/01/23/leaflet-0-7-vs-openlayers-3-beta-1-on-ios/
[2011 September] UPDATE : new Openlayers 2.11 works quite well on iPad/iPhone.
I have experimented with OpenLayers 2.9 using touch.js extension for capturing touch events from iPhone/iPad. Although stripped down version of OL is about 184KB, the performance is very bad. If you have iPhone/iPad check this experiment: http://www.sumbera.com/lab/iphone/katastr.htm (note it will not work from desktop browser)
OL team is working on v3 of OL to be more lighter, faster even for mobile devices, see here: http://trac.osgeo.org/openlayers/wiki/three or here : http://openlayers.org/blog/2010/06/30/openlayers-3-on-github/
Meanwhile I have tested new GoogleMaps v3 how it works on iPhone/iPad. despite few problems (stability, problems with cached tiles or disconnected JavaScirpt to load new tiles -iPad) it works pretty well . Check yourself this page from your mobile touch device (iPhone.iPad,Android): http://www.ikatastr.cz/iKatastrM.htm
Conclusion : Google Maps v3 wins on mobile over the Openlayers 2.9