The only problem with having Dalvik on your /cache partition (internal memory) is that it can interfere with Market Updates if the Dalvik size grows too large. Every time your Market updates, it writes a "Download.apk" file to your /cache. If you prefer to keep your Dalvik on your /cache (makes more rooms for app on /data), you can simply use Root Explorer to delete the "Download.apk" files from your /cache after a Market update is complete (they are completely unnecessary).