I think the problem with the current planet splitting tools like (osmosis and osmcut) is that it requires a lot of preprocessing or RAM to gather all the road polylines. Determining the intersection with a bbox when you have the polylines available is CPU intensive (but doable), but I guess the OSM XML format isn’t very suitable to read polylines from.
I see three possible solutions:
Load all ways into memory (lots of RAM needed) and check each node to see to which way it belongs (CPU intensive), then save the road/node combination as polyline. Finally check each polyline agains every tile bbox to determine if crosses the border. Then add a node on the border position. Then split the way on the border node and save the resulting ways in the individual tile files. That is a lot of work.
Do as described above, but convert the planet file into a shape file first using osm2shp (dunno what how much RAM/CPU time that requires).
While writing this I forgot the third option I had in mind
Edit: I cannot find any OSM to Shapefile converter in the SVN repository, so this leaves us at the first option…