if you don't care about the warranty just open it and put some heatsink on the cpu and other heated chips
you can find heatsinks on overclocker's sites like the one used to cool chipsets or rams or vga rams
some of them even got conductive tape sticked on the bottom so you don't even need to put the conductive grease on it...
just google something like ram heatsink or mosfet heatsink...I think you can find ram heatsinks in any computer shop for about 15-20$
the copper one are "most efficent" and expensive than the alluminium one....just mind their dimensions and the space you have on the guruplug before buying it!!
another way to take away a lot of heat from the guruplug is to take out the standard power supply and put it far from the guruplug boards so the PSU heat will not heat the boards...you can even use an external psu like the one you use for cell phones or routers...you only need to respect voltage and power requirements....for the sheevaplug you need a 5V 3A psu...I think the pogoplug runs on 5v and will not draw more than 4 amperes...maybe buy a 5A psu if you want no surprise.
if there's still to much heat you can plug to the 5V psu a 12x12cm 12v computer fan so it'll run very very slowly and quiet but still blow a lot of air on the heatsinks....but I think that copper heatsinks and a fan will cool down a normal CPU so it'll be kinda overkill...
obviously the smart and good-looking but unstable pogoplug will become an ugly opened pogoplug with an external psu and heatsinks all around but will run for years on full load with no heat problems...