I have found a little problem with running Win98 on a Win7 host, and I also have the solution.
To share data between guest and host it is necessary to use network drives. I am using a bridged network connection, but I can see no reason why other network connections shouldn't work.
To share the Win98 drive with Win7 is very easy. Simply set up the Win98 networking to use the same workgroup as Win7, then share the drive in the normal way (giving full access rather than the default read only as required) and Win7 will be able to find the Win98 guest and access the drive.
To share the Win7 drive with Win98 is more complicated. First, Win7 has to have its network settings configured to be Win98 compatible. This is the easy bit. Simply go to the advanced sharing settings for the Home or Work network and turn Password Protected Sharing off. This should enable Win98 to see the Win7 host, although you may also need to adjust the Win7 firewall to allow VirtualBox through on the Home or Work network. At this point you can set up the Win7 drive to be shared (using advanced sharing and ensuring permissions are set using the Everyone account with access rights as required) and then you are ready to access the host machine drive from Win98. Unfortunately, Win7 doesn't understand the Win98 request to browse the host when viewing it in Network Neighborhood, so you can't get to anything on the host. However, the solution is to type in the full path of the shared drive on the host into the Network Neighborhood (or Win Explorer) address bar in the format \\HostPCName\SharedDrive, and you will be into the drive able to navigate it normally.
If you want to make things a little easier to access in the future, you can map the shared drive to a drive letter. This is a little harder than usual as you cannot get into the host machine and therefore cannot do it by right clicking on the shared folder to assign the drive letter. Again, this can be overcome by running Windows Explorer and using the Map Network Drive under the Tools menu. Select the drive letter required, then type in the path for the shared folder (same format as above), tick the reconnect button if you want it to be permanent, hit OK and you're in business. Please note that this option is only available in Windows Explorer. My Computer and Network Neighborhood windows, although very similar, don't have the Tools menu needed to map the drive.