Hmm, interesting. I've been giving the subject some thought myself recently (I have a Sakura) and I've come to the conclusion that it would be an interesting if expensive experiment.
Let's start with your energy requirements. Your watt-hours per mile will be the most important part of determining how much power you need. Basically ,
watt-hours per mile = (battery voltage x batt amp-hours) / range
so for my Sakura
36v x 14 amp-hours = 504 watt-hours.
Range is 15 miles, so
504 watt-hours / 15 miles = 33.6 Wh/mile.
So to charge up less than a mile's worth of juice per hour of sunlight (maybe a lot less depending on charge losses and how much the panels are in shade when charging) you'll need 20 or 30 watts of panels, usually these come in ten-watt increments. You'll want flexible ones or you won't be able to carry the panels' own weight. Here is one:
http://cgi.ebay.co.uk/10W-Flexible-ICP- ... p1638.m122
Electric bikes usually come in either 24volt for the bycicle type ones, and 36v for the more moped style things, so if you've got the moped type thing you'll need three panels. At Ã‚Â£83 each + postage that could run into money, but if you want to go touring on your electric bike it might well be worth it.
As you can see, even in midsummer you aren't going to get that far on a day's sunlight, but you could do, say, a electric powered cycling holiday on it and maybe it could indeed extend your range day-to-day to the point where you don't have to carry a charger around with you. I'd certainly like to find out!
It all depends on what your application is, really. If you just want better range, consider upgrading your battery to a more efficient chemistry first. Remember, the solar panels obviously don't work at night, and will be far more efficcient in summertime. On the other hand, Lithium batteries are getting quite cheap now. In fact, your bike already uses the best battery type available (and one that can be dangerous to use under non-standard charging regimes too), so you're looking at changing your bike before you do anything, in any case.
Then again, if you want to travel indefinitely without plugging into the mains, ie for holidaying purposes, obviously experimenting with solar would be much more useful - in fact that's very much something I'm interested to hear about, so go for it and write up your experience here!