What do you mean by smoothly? Are there any issues that aren't solved by adding the VirtualBox client tools? What other distributions have you had a problem with?
I haven't run into any issues on the client side using many new and older versions of many distributions of linux.
Side note: On the server side the newest Ubuntu Karmic kernel has issues with NMI. This can be solved with an alternate kernel for now (currently Karmic is an alpha release). Other distros with 2.6.30+ kernels don't have the bug.
Treat Virtuals as you would remote machines. There are many choices.
Text mode for virtuals if possibly, lamp servers, etc.. No X installed on virtual. If tuned well this will give maximum performance.
Run X apps over virtual network, still no X on virtuals.
Use seamless mode, launch over RDP using scripts/menu entries and use the lightest window manager possibly (enter argument over which is the better light-weight window manager)
Use light-weight window manager for virtuals like Openbox, IceWM, Windowmaker or even LXDE, XFCE, Flux, etc..
Use an NX client and nx server like Google's neatx. More applicable to remote virtual access.
Use all the same performance tuning tips you would use for an linux audio workstation or older hardware running linux. In the virtual client, disable or turn off auto-everything; dhcp, zeroconf, hal hardware detection, cups printing service, bluetooth, acpi, pulse, etc..
As stated above, building up rather than installing a "desktop" distribution is the best.
In the next few years, perhaps even now, you should be able to use an appliance or cloud distribution like Ubuntu's JEOS or Fedora's AOS, etc.. (They're not ready yet, IMHO. Great ideas, looking forward to improvements in them.)