Yesterday CNET’s Charles Cooper reported that Twitter had signed a term sheet to raise $15 – $20 million in a new round of venture funding

It’s fairly well known that they are out talking to VCs (it’s hard to keep that quiet), but our sources are saying no deal is imminent, and that Twitter is still meeting with potential suitors.

The company has received a number of term sheets, according to one source. Valuations range from $60 million on the low end to more than $150 million on the high end. There is, clearly, a bit of salivating going on around Twitter.

One reason for all the attention: growth rates suggest that it is now just a matter of when, not if, Twitter usage will go mainstream. Twitter usage has “exploded” in the last couple of months, says our source (this is supported by Comscore, which shows total page view doubling from 10 to 20 million between February and March, although it is nearly impossible to guess at total Twitter usage since there are no many access points other than the Twitter website, via the API).

The company may not take the highest valuation offered – it limits potential exits and makes hiring harder. But it seems likely that the round will be done at a valuation above $60 million.

Twitter now has 17 or so employees. Their biggest non-payroll expense is supposedly SMS fees, although they have hit a ceiling and no longer pay a marginal fee per message as the company grows.